Add tests/ to mypy scope; fix real type gaps it surfaces

[derek73]

Summary

• mypy currently only checks nameparser/ (packages = ["nameparser"]), so tests/ has never been type-checked. Running mypy against tests/ directly surfaced 31 errors.
• Most were real, narrow-but-fixable annotation gaps in the library, not bugs in the tests:
  • Constants.string_format / Constants.suffix_delimiter can be None (already documented/used that way) — annotated str | None.
  • is_prefix / is_conjunction / is_suffix already handle a list[str] argument at runtime (isinstance(piece, list)) but were typed piece: str — widened to str | list[str].
  • HumanName.__setitem__ delegates to _set_list, which accepts str | list[str] | None, but was typed value: str — widened to match.
  • SetManager.add_with_encoding already decodes bytes input at runtime but was typed s: str — widened to str | bytes.
  • test_override_capitalization_exceptions reused a regex-valued TupleManager (meant for regexes) as a stand-in for capitalization_exceptions (which is str -> str) — swapped in a semantically correct value instead of suppressing.
  • A local snap = {} in test_constants.py needed an explicit dict[str, Any] annotation since it holds heterogeneous values across branches.
• The remaining handful of errors are tests that deliberately violate the type contract to assert a TypeError or exercise a documented None-disables-the-feature toggle (e.g. empty_attribute_default = None, regexes.emoji = False). Added # type: ignore[code] on those lines, matching the existing convention already used elsewhere in test_constants.py.
  • empty_attribute_default specifically was not widened to str | None like its siblings above, even though None is documented/supported there too — doing so would cascade into changing ~8 public property return types (title, first, middle, last, suffix, nickname, initials()) from str to str | None, a bigger API decision than this PR's scope. Left as # type: ignore[assignment] at each site, with a comment explaining the reasoning.
• Added tests to [tool.mypy] packages in pyproject.toml so CI's uv run mypy now covers it.
• Updated AGENTS.md: the Commands section (uv run mypy nameparser/ → uv run mypy) and a Gotcha that this PR's own change made stale (it previously said mypy intentionally excludes tests/).

Test plan

• mypy — 0 errors, 35 source files checked
• ruff check — all checks passed
• pytest — 1438 passed, 4 skipped, 22 xfailed
• Reviewed via code-reviewer, pr-test-analyzer, and comment-analyzer subagents — no correctness issues found; one pre-existing weak-assertion test noted as a separate follow-up, not addressed here

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.52%. Comparing base (94b253c) to head (b32b65c).
❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #250   +/-   ##
=======================================
  Coverage   97.52%   97.52%           
=======================================
  Files          13       13           
  Lines         888      888           
=======================================
  Hits          866      866           
  Misses         22       22           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.