fix(http2): tolerate stream errors after response completion

[rupayon123]

Summary

Fixes #922. When a streaming request has already delivered its response end, a later HTTP/2 stream error can reach failRequest after the handler cleared its request. The handler now safely skips the second request failure while still closing the HTTP/2 channel and clearing timeout state.

Reproduction and validation

• Reproduced the attached localhost-only HTTP/2 reset scenario against the unmodified upstream checkout; it crashed at the force unwrap in HTTP2ClientRequestHandler.
• The same attached reproduction completed normally against this branch.
• Added an EmbeddedChannel regression for a streamed request, successful response end, then a later stream error.
• swift test --filter HTTP2ClientRequestHandlerTests/testErrorAfterResponseEndWhileRequestBodyIsStreamingDoesNotCrash passed.
• swift test --filter HTTP2ClientRequestHandlerTests passed.
• swift format lint --strict --configuration .swift-format ... and git diff --check passed.

The attached end-to-end reproducer uses a localhost test certificate and does not contact an external host. The unit test verifies handler state and channel closure; it does not replace the end-to-end reproduction.

AI assistance: OpenAI Codex assisted with the investigation and draft. I reviewed the diff and ran the checks listed above.