fix(core): classify copilot utility requests - #41840

Summary

• route title generation and compaction through the existing session HTTP plugin hooks
• classify GitHub Copilot title requests as X-Interaction-Type: conversation-background
• classify automatic and manual compaction as X-Interaction-Type: conversation-compaction
• keep provider-specific classification entirely in the GitHub Copilot plugin
• share the session HTTP hook middleware across normal model execution, title generation, and compaction

Testing

• bun test test/session-compaction.test.ts test/session-compact.test.ts test/plugin/provider-github-copilot.test.ts test/session-title.test.ts test/session-model-request.test.ts from packages/core
• clean declaration build: rm -rf node_modules/.ts-dist && bun typecheck from packages/core
• monorepo pre-push typecheck is blocked locally by unrelated missing clipboard exports from the installed @opentui/core in packages/tui