refactor: remove dead code identified in issue #4 (~467 lines)

[Vattsa-11]

Remove confirmed-dead subsystems that are never instantiated or called:

• AutoDocRouter class + create_auto_router() from auto_docs.py
• decorators_style.py in full (get/post/put/delete/patch/Path/Body decorators that only attach metadata, never register routes)
• Router.dynamic_patterns (never populated; generator.py loops method_trees and route_cache instead)
• DependencyCache class + cleanup_caches() + inject() from depends.py (resolve_dependencies uses a plain dict on request.state)
• HandlerProtocol from router.py (defined, never used as a type)
• create_model() from validation/models.py (zero callers)

Also cleaned up now-unused imports: Protocol in router.py, TypeVar in depends.py, Callable in auto_docs.py, and AutoDocRouter/decorators_style re-exports from init files.

All 252 tests pass. Ruff clean.

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