### What problem does this PR solve?
Fixes#15840.
The Go HTTP server sets `WriteTimeout: 120s`, which also applies to
long-lived SSE responses. Existing Go streaming handlers did not clear
the per-response write deadline, so streams that run longer than the
server timeout can be terminated mid-response.
This PR adds a small handler helper that clears the response write
deadline for SSE requests and calls it only in existing Go streaming
branches:
- conversation completion streaming
- provider chat streaming
- provider transcription streaming
- provider speech streaming
The global server `WriteTimeout` remains unchanged for non-streaming
requests.
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
### Test plan
- `/root/go/bin/go test ./internal/handler -run
TestDisableWriteDeadlineForSSEAllowsLongLivedStream -count=1`
- `/root/go/bin/go test ./internal/handler -count=1`
