fix: Rename shared HttpCompatApi.scala to fix release build (#450)

## Summary
- Rename shared `uni/src/main/scala/wvlet/uni/http/HttpCompat.scala` to
`HttpCompatApi.scala` to avoid duplicate entry conflict with
platform-specific `HttpCompat.scala` files during `packageSrc`
- Same root cause as #449

## Test plan
- [x] `./sbt uniJVM/publishSigned` succeeds locally
- [ ] CI passes
- [ ] After merge, re-tag and verify release pipeline completes

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

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: xerial