This is a benchmark created by @rafi. It demonstrates `errors.Is`
is very inefficient when the reference error does not match the
input error.

Previously, `errors.Is` was very inefficient if the reference error did
not match any errors in the chain.

There were two significant sources of inefficiency:
1. The code would pessimistically construct the error mark for every
   error in the input error chain. This is O(chain_length^2). This was
   a lot of unnecessary allocations and caused the runtime to be O(n^2)
   in the average case instead of O(n).
2. The code compared the `Error()` message before comparing the types.
   It is possible to compare error types with zero allocations whereas
   computing the message often requires an allocation.

errorbase: optimize errors.Is

Also fix a non-constant format string in fmttests that the Go 1.26
vet checker now flags.

Co-Authored-By: roachdev-claude <roachdev-claude-bot@cockroachlabs.com>

build: bump min Go version to 1.25, add 1.26 to CI

sentry-go v0.46.0 removed the Extra field from sentry.Event (see
getsentry/sentry-go#1274). ReportError merged its extraDetails map into
event.Extra, which now fails to compile against sentry-go >= v0.46.0.

Because report_api.go in the root errors package imports the report
sub-package, the typecheck failure blocks the entire module for every
downstream consumer -- not just callers of the Sentry reporting code.

Switches the per-key merge to event.Contexts, which is the documented
replacement. Each extra is stored under sentry.Context{"value": v} to
conform to the Context schema (map[string]interface{}).

Along the way:

- Adds the new Close() and FlushWithContext(ctx) methods required by the
  v0.46.0 sentry.Transport interface on the test-only interceptingTransport
  in report/report_test.go and fmttests/datadriven_test.go.
- In the fmttests printer, filters to only Contexts entries that carry
  our "value" sub-key, so Sentry's built-in default contexts
  (device/os/runtime/trace) emitted by sentry.NewEvent() do not leak into
  fixtures. Preserves the "== Extra" label -- net fixture diff is empty.
- Fixes a pre-existing non-constant-format-string call in
  fmttests/datadriven_test.go flagged by the stricter go vet in the Go
  toolchain now required by sentry-go v0.46.0.
- Bumps the go directive to 1.25.0 (drops the no-longer-needed toolchain
  line) because sentry-go v0.46.0 itself declares go 1.25.0.

Fixes #158

report: migrate Event.Extra to Event.Contexts (sentry-go v0.46+ compat)