…#3052)

## Summary

The postgres source built its connection URL with `fmt.Sprintf("%s:%s",
host, port)`, which produces invalid authority for IPv6 literals. pgx
then parsed the result as an 8-segment IPv6 with the port absorbed as a
trailing hextet, producing dial errors such as:

```
dial tcp [fd00::1:5432]:5432: connect: no route to host
```

Replacing the concatenation with
[`net.JoinHostPort`](https://pkg.go.dev/net#JoinHostPort) wraps IPv6
literals in brackets as required by RFC 3986, and leaves IPv4 addresses
and hostnames unchanged.

URL construction is extracted into a new `BuildPostgresURL` helper so it
can be unit-tested without a live database.

## Relationship to googleapis#3020

googleapis#3020 ("apply URL encoding to query string params") merged to `main`
while this PR was open. It applied the same `url.Values.Encode()` fix
directly inside `ConvertParamMapToRawQuery`. This PR independently
introduced the same encoding improvement but inlined it into the new
`BuildPostgresURL` helper and removed `ConvertParamMapToRawQuery` as no
longer needed (the postgres package was its only caller;
`internal/sources/cockroachdb` has its own independent copy and is
unaffected). After rebasing on `main`, the conflict was resolved in
favor of this PR's version, which is a strict superset: same
URL-encoding behavior plus IPv6 support and a testable builder.

## Changes

- `internal/sources/postgres/postgres.go`
  - Introduce `BuildPostgresURL` that uses `net.JoinHostPort`.
- Encode query parameters with
[`url.Values.Encode()`](https://pkg.go.dev/net/url#Values.Encode) (per
gemini-code-assist review): proper escaping of `&`, `=`, spaces, and
deterministic key order. Equivalent behavior to googleapis#3020, encapsulated in
the new helper.
- Remove the now-unused `ConvertParamMapToRawQuery` helper. It was only
referenced from this file; `internal/sources/cockroachdb` has its own
independent copy and is not affected.
- `internal/sources/postgres/postgres_test.go`
- Add `TestBuildPostgresURL` covering hostname, IPv4 (`127.0.0.1`), IPv6
loopback (`::1`), IPv6 documentation (`2001:db8::1`), IPv6 link-local
with zone id (`fe80::1%eth0`), query-parameter key ordering, and
URL-encoding of values containing special characters. Each expected URL
is also fed through `pgx.ParseConfig` to verify downstream
compatibility.
  - Drop `TestConvertParamMapToRawQuery` alongside the removed function.

## Backward compatibility

- `net.JoinHostPort` behaves identically to `fmt.Sprintf("%s:%s", host,
port)` for hostnames and IPv4 addresses — brackets are only added when
the host contains a colon.
- The query-string output order is now deterministic (sorted by key)
rather than map-iteration order. The previous order was already
non-deterministic, so any consumer that depended on a specific order was
already broken.
- `queryParams` values that previously contained special characters such
as `&` or `=` would have produced malformed URLs; they are now correctly
percent-encoded (same fix as googleapis#3020).

## Test plan

- [x] `go test -race ./internal/sources/postgres/...` — all tests pass
including new IPv6 and query-param encoding cases.
- [x] `go vet ./internal/sources/postgres/...` — clean.
- [x] End-to-end: built the patched toolbox and used it via `--prebuilt
postgres --stdio` to connect to a real PostgreSQL instance reachable
only over IPv6 (ULA address via a WireGuard-style tunnel). The MCP
client listed tools and executed queries successfully, whereas the
unpatched binary failed with the mangled dial error shown above.
- [x] Rebased onto current `main` (post-googleapis#3020); conflicts resolved
cleanly, all tests green.

---------

Co-authored-by: Yuan Teoh <45984206+Yuan325@users.noreply.github.com>