Seen when running the macOS clang-tidy job with test bundles disabled:
```
tests/libtest/libauthretry.c:96:12: error: The value '126' provided to the cast
  expression is not in the valid range of values for the enum
  [clang-analyzer-optin.core.EnumCastOutOfRange,-warnings-as-errors]
   96 |     return TEST_ERR_MAJOR_BAD;
      |            ^
tests/libtest/test.h:99:32: note: expanded from macro 'TEST_ERR_MAJOR_BAD'
   99 | #define TEST_ERR_MAJOR_BAD     (CURLcode) 126
      |                                ^~~~~~~~~~~~~~
include/curl/curl.h:519:9: note: enum declared here
[...]
```
Ref: https://github.com/curl/curl/actions/runs/13909934306/job/38921798619?pr=16750#step:14:384

Follow-up to fabfa8e curl#15825

Seen when running the macOS clang-tidy job with test bundles disabled:
```
tests/libtest/libauthretry.c:96:12: error: The value '126' provided to the cast
  expression is not in the valid range of values for the enum
  [clang-analyzer-optin.core.EnumCastOutOfRange,-warnings-as-errors]
   96 |     return TEST_ERR_MAJOR_BAD;
      |            ^
tests/libtest/test.h:99:32: note: expanded from macro 'TEST_ERR_MAJOR_BAD'
   99 | #define TEST_ERR_MAJOR_BAD     (CURLcode) 126
      |                                ^~~~~~~~~~~~~~
include/curl/curl.h:519:9: note: enum declared here
[...]
```
Ref: https://github.com/curl/curl/actions/runs/13909934306/job/38921798619?pr=16750#step:14:384

Follow-up to fabfa8e curl#15825

Seen when running the macOS clang-tidy job with test bundles disabled:
```
tests/libtest/libauthretry.c:96:12: error: The value '126' provided to the cast
  expression is not in the valid range of values for the enum
  [clang-analyzer-optin.core.EnumCastOutOfRange,-warnings-as-errors]
   96 |     return TEST_ERR_MAJOR_BAD;
      |            ^
tests/libtest/test.h:99:32: note: expanded from macro 'TEST_ERR_MAJOR_BAD'
   99 | #define TEST_ERR_MAJOR_BAD     (CURLcode) 126
      |                                ^~~~~~~~~~~~~~
include/curl/curl.h:519:9: note: enum declared here
[...]
```
Ref: https://github.com/curl/curl/actions/runs/13909934306/job/38921798619?pr=16750#step:14:384

Follow-up to fabfa8e curl#15825

- cmake: disable test bundles for clang-tidy builds.
  clang-tidy ignores #included .c sources, and incompatible with unity
  and bundles. It caused clang-tidy ignoring all test sources. It also
  means this is the first time tests sources are checked with
  clang-tidy. (autotools doesn't run it on tests.)

- cmake: update description for `CURL_TEST_BUNDLES` option.

- fix tests using special `CURLE_*` enums that were missing from
  `curl/curl.h`. Add them as reserved codes.

- fix about ~50 other issues detected by clang-tidy: unchecked results,
  NULL derefs, memory leaks, casts to enums, unused assigments,
  uninitialized `errno` uses, unchecked `open`, indent, and more.

- drop unnecessary casts (lib1533, lib3207).

- suppress a few impossible cases with detailed `NOLINT`s.

- lib/escape.c: drop `NOLINT` no longer necessary.
  Follow-up to 72abf7c #13862 (possibly)

- extend two existing `NOLINT` comments with details.

Follow-up to fabfa8e #15825

Closes #16756

build:
- autotools: fix to build generated sources for the `tidy` target.
- autotools: allow passing custom clang-tidy options via
  `CURL_CLANG_TIDYFLAGS` env.
- cmake: add `CURL_CLANG_TIDY` option to configure for `clang-tidy`.
  Also add:
  - `CLANG_TIDY` variable to customize the `clang-tidy` tool.
  - `CURL_CLANG_TIDYFLAGS` to pass custom options to `clang-tidy`.
- apply `--enable-werror` and `-DCURL_WERROR=ON` to `clang-tidy`.

CI/GHA:
- add clang-tidy job for Linux, using autotools and clang-tidy v18.
  This one needs to disable `clang-analyzer-valist.Uninitialized`
  to avoid false positives:
  llvm/llvm-project#40656
  Duration: 5.5 minutes
- add clang-tidy job for macOS, using cmake and clang-tidy v19.
  This one also covers tests and examples, and doesn't hit the false
  positives seen with llvm v18 and earlier.
  Duration: 4.5 minutes
- Linux/macOS: skip installing test dependencies when not building or
  running tests.

fix fallouts reported by `clang-tidy`:
- lib:
  - cf-h2-proxy: unused assignment in non-debug builds.
  - cf-socket: silence warning.
    FIXME: curl#15825 (comment)
  - ftp: NULL passed to `strncmp()`.
  - http2: NULL-ptr deref.
  - mprintf: silence warning.
- src/tool_writeout: NULL passed to `fputs()`.
- examples:
  - invalid file pointers.
  - missing `fclose()`.
- tests:
  - http/clients/hx-download: memory leaks on error.
  - http/clients/hx-download: memory leak on repeat `-r` option.
  - server: double `fclose()`.
    https://www.man7.org/linux/man-pages/man3/fclose.3.html
  - server: invalid file pointer/handle.
  - server/getpart: unused assignments.
  - server/mqttd: leak on failed `realloc()`.
  - server/tftpd: NULL passed to `strcmp()`.

Closes curl#15825

- cmake: disable test bundles for clang-tidy builds.
  clang-tidy ignores #included .c sources, and incompatible with unity
  and bundles. It caused clang-tidy ignoring all test sources. It also
  means this is the first time tests sources are checked with
  clang-tidy. (autotools doesn't run it on tests.)

- cmake: update description for `CURL_TEST_BUNDLES` option.

- fix tests using special `CURLE_*` enums that were missing from
  `curl/curl.h`. Add them as reserved codes.

- fix about ~50 other issues detected by clang-tidy: unchecked results,
  NULL derefs, memory leaks, casts to enums, unused assigments,
  uninitialized `errno` uses, unchecked `open`, indent, and more.

- drop unnecessary casts (lib1533, lib3207).

- suppress a few impossible cases with detailed `NOLINT`s.

- lib/escape.c: drop `NOLINT` no longer necessary.
  Follow-up to 72abf7c curl#13862 (possibly)

- extend two existing `NOLINT` comments with details.

Follow-up to fabfa8e curl#15825

Closes curl#16756

Skip clang-tidy while compiling curlu and curltool internal libraries.
To save about 1 minute per run. These libraries compile the lib and src
sources a second time, with the `UNITTESTS` macro enabled, which makes
tiny difference, for internal use. I figure it's not worth the extra CI
(and local) time because finding extra issues in these passes is
unlikely, and if found, not critical.

autotools also doesn't check curlu and curltool with clang-tidy.

Ref: #17680 (comment)
Ref: https://stackoverflow.com/questions/61867616/ignore-certain-files-when-using-clang-tidy
Ref: https://cmake.org/cmake/help/latest/prop_tgt/LANG_CLANG_TIDY.html

Follow-up to fabfa8e #15825

Closes #17693

Skip clang-tidy while compiling curlu and curltool internal libraries.
To save about 1 minute per run. These libraries compile the lib and src
sources a second time, with the `UNITTESTS` macro enabled, which makes
tiny difference, for internal use. I figure it's not worth the extra CI
(and local) time because finding extra issues in these passes is
unlikely, and if found, not critical.

autotools also doesn't check curlu and curltool with clang-tidy.

Ref: curl#17680 (comment)
Ref: https://stackoverflow.com/questions/61867616/ignore-certain-files-when-using-clang-tidy
Ref: https://cmake.org/cmake/help/latest/prop_tgt/LANG_CLANG_TIDY.html

Follow-up to fabfa8e curl#15825

Closes curl#17693