fix: address PR review comments on migration docs and type hints

- Fix all migration.md examples to use ServerRequestContext instead of RequestContext
- Fix all imports to use 'from mcp.server import Server, ServerRequestContext'
- Apply ruff formatting to code examples
- Add Server constructor calls to examples that were missing them
- Re-export ServerRequestContext from mcp.server.__init__
- Add type hints to TaskResultHandler docstring example