Refactor: Split connection.go into focused, maintainable files

[Dumbris]

Summary

Refactors the massive 3,587-line connection.go file into 7 focused, maintainable files, achieving a 92% reduction in the main file size while maintaining 100% test coverage and zero breaking changes.

Fixes #273

Changes

File Structure

File	Lines	Purpose
connection.go	275 (-92%)	Main Connect() dispatcher, core types
connection_browser.go	95	Browser launching, GUI detection
connection_docker.go	145	Docker isolation setup and helpers
connection_stdio.go	451	Stdio transport and command building
connection_http.go	314	HTTP/SSE transports and auth strategies
connection_oauth.go	2,189	All OAuth flows and error handling
connection_lifecycle.go	243	Initialization and disconnect logic
Total	3,712

Key Improvements

• Maintainability: Each file has a single, clear responsibility
• Readability: Functions are grouped logically, easier to navigate
• Testability: Related code is colocated with its tests
• Scalability: Future changes can target specific files
• No Breaking Changes: All public APIs preserved

Dependency Hierarchy

connection.go (dispatcher)
    ├─→ connection_stdio.go
    │       └─→ connection_docker.go
    ├─→ connection_http.go
    │       └─→ connection_oauth.go
    │               └─→ connection_browser.go
    └─→ connection_lifecycle.go

Testing

✅ All unit tests pass (100+ tests)
✅ Race detection clean (go test -race)
✅ Linter passes with no errors
✅ OAuth E2E tests pass
✅ No circular dependencies

Commits

Each phase of the extraction was committed separately for reviewability:

1. Extract browser utilities (95 lines)
2. Extract Docker isolation (145 lines)
3. Extract stdio transport (451 lines)
4. Extract OAuth flows (2,189 lines)
5. Extract HTTP/SSE transport (314 lines)
6. Extract lifecycle functions (243 lines)
7. Final verification and documentation

Review Notes

• All OAuth state fields remain in Client struct (in client.go)
• No changes to function signatures or behavior
• Test files remain in same package with full access
• Constants are preserved in appropriate files

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