chore: tighten mypy strict mode and remove dead code

Enable warn_unreachable, extra_checks, local_partial_types in pyproject.
Remove dead defensive branches and AI-slop union members; replace narrow
band-aid type:ignore with proper signature widening or targeted ignores
for genuine runtime-defensive paths (double-checked locking, hook misuse,
unfollowed-import boundaries).

pyproject.toml

@@ -124,8 +124,11 @@ disallow_any_unimported = true
 no_implicit_optional = true
 check_untyped_defs = true
 warn_return_any = true
+warn_unreachable = true
 show_error_codes = true
 warn_unused_ignores = true
+local_partial_types = true
+extra_checks = true
 python_version = "3.12"
 exclude = "(?x)(^lib/crewai/src/crewai/cli/templates/|^lib/cli/src/crewai_cli/templates/|^lib/crewai/tests/|^lib/crewai-tools/tests/|^lib/crewai-files/tests/|^lib/cli/tests/|^lib/devtools/tests/)"
 plugins = ["pydantic.mypy"]