fix(mcp): keep GET SSE responses uncompressed

[RaviTharuma]

Summary

• mark GET MCP event streams no-cache, no-transform so Next native compression cannot buffer SSE frames
• preserve JSON/non-SSE responses and existing encodings so normal gzip behavior remains available
• add regression coverage for gzip negotiation variants, header preservation, and prompt first-event delivery

Tests

• node --import tsx/esm --test --test-reporter=spec tests/unit/mcp-sse-response-headers-8277.test.ts
• node --import tsx/esm --test --test-reporter=spec tests/unit/mcp-session-sweep.test.ts
• npm run typecheck:core
• npx eslint open-sse/mcp-server/httpTransport.ts tests/unit/mcp-sse-response-headers-8277.test.ts

Closes #8277
Related #6736

🤖 Generated with Claude Code

[diegosouzapw]

Thanks, @RaviTharuma! 🙏 Closing — the branch is catastrophically drifted from the current release tip (540 files / +35k/-29k, almost all base-drift), so the actual 'keep GET SSE responses uncompressed' change is buried and can't be merged safely. A fresh focused branch against the latest release/v3.8.49 (just the MCP GET SSE compression skip) would merge in seconds — please re-open that way and I'll fast-track it.