fix: validate REST request timeout header (#50195)

fixes: #49890

This PR validates the REST `Request-Timeout` header instead of silently
ignoring invalid values.

Changes:
- Return `ErrParameterInvalid` when `Request-Timeout` is present but
cannot be parsed as an integer.
- Preserve the underlying `strconv.ParseInt` error in the response
message.
- Add timeout middleware coverage for invalid values such as `3.5` and
`abc`.

Test:
- `go test -tags dynamic,test -gcflags="all=-N -l" -count=1
./internal/distributed/proxy/httpserver -run 'TestTimeoutMiddleware'`
*(fails locally because generated C/CGo headers are unavailable:
`storage/loon_ffi/external_spec_c.h`, `C.CArrowReaderConfig`)*

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Signed-off-by: xiaofanluan <xiaofan.luan@zilliz.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>