fix(githubevents): runtime hardening (data race, error wrapping, go floor)

[cbrgm]

What

Three runtime-hardening fixes from the post-merge review of main.

• Fix a data race on the handler maps. Writes (On*/SetOn*) took mu.Lock, but the handle* read paths read the maps with no lock, so registering a handler while an event was being dispatched raced on the map (the RWMutex only ever guarded writes). The read paths now RLock, copy the handler slices, and RUnlock before dispatching, so a callback can still register mid-dispatch without deadlocking.
• Wrap request errors. HandleEventRequest used %s + a trailing \n, so callers couldn't errors.Is/As the underlying go-github error. Now %w, no newline.
• Lower the go directive from the 1.26.5 patch-pin (chore(deps): update go module directive to v1.26.5 #275) to 1.25.0, the actual floor both go-github v90 and x/sync require. .go-version still drives the CI toolchain.

Adds a -race test and a make test-race CI step so this class of bug is caught going forward.

Why

The race is the real one: the RWMutex looked like it protected reads but its RLock was never called, so anyone registering handlers concurrently with serving could hit fatal error: concurrent map read and map write. The snapshot-under-RLock pattern keeps the lock off the callback execution path (holding it there would deadlock a callback that registers). Behavior is otherwise unchanged: same handlers dispatched, same order, same first-error-wins and panic-recovery semantics. Public API is untouched (apidiff confirms).

Testing

$ CGO_ENABLED=1 go test -race ./githubevents/     # new TestNoDataRace* : was DATA RACE, now ok
ok  github.com/cbrgm/githubevents/v2/githubevents

$ make test        # full suite green, coverage 85.5%
$ make apidiff      # >> API compatible (go-github version-only changes ignored)
$ golangci-lint run # 0 issues

The -race test fails on main (concurrent map read/write) and passes here.

Checklist

• Tests added/updated (race_test.go, run under -race in CI via make test-race)
• No breaking changes (unexported internals + error wrapping; apidiff compatible; go floor lowered, not raised)
• Readable commit history (3 commits: race fix, error wrap, go directive)
• AI code review considered and comments resolved