fix(types): correct nullability of config fields in command TypedDicts

[bearomorphism]

Description

Several command TypedDicts declared fields as non-nullable (int, str, dict, list) while argparse produces None when CLI flags are omitted. Runtime code already handles None correctly (via or fallbacks, .get(), explicit None checks), but the type annotations were misleading to both developers and mypy.

Changes

• CommitArgs: message_length_limit: int → int | None
• CheckArgs: message_length_limit: int → int | None, allowed_prefixes: list[str] → list[str] | None
• BumpArgs: file_name: str → str | None
• ChangelogArgs: file_name, start_rev, tag_format, version_scheme, export_template, template → add | None; extras → dict[str, Any] | None

Scope

Type annotations only — no behavioral changes.

Related

• Fix: message_length_limit config is ignored when CLI flag is unset (commit/check) #1899 — the message_length_limit config-ignored bug (this PR makes the types accurate; the behavioral fix is in Fix and improve test: message_length_limit #1900)
• Fix and improve test: message_length_limit #1900 — behavioral fix for message_length_limit precedence (complementary, no conflict)
• fix: try to use ChainMap to chain settings, fix message_length_limit inconsistency #1812 — ChainMap-based settings refactor (complementary, no conflict)
• Use ChainMap to rewrite the cmd options <- configuration file <- default configuration chain #1672 — tracking issue for the settings chain redesign

Checklist

• I have read the contributing guidelines

Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

Generated-with: GitHub Copilot CLI following the guidelines

Code Changes

• Add test cases to all the changes you introduce
• Run uv run poe all locally to ensure this change passes linter check and tests
• Manually test the changes
• Update the documentation for the changes

No test or doc changes needed — this is a pure type annotation fix with no behavioral impact.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.23%. Comparing base (4b93a50) to head (bef33f7).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1952   +/-   ##
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  Coverage   98.23%   98.23%           
=======================================
  Files          61       61           
  Lines        2779     2779           
=======================================
  Hits         2730     2730           
  Misses         49       49           

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