Improve mapping type completeness

[heejaechang]

Summary

Closed after policy review. The proposed completeness gains depended on reparameterizing declarations that already carried explicit # type: ignore[type-arg] ownership boundaries, adding checker-specific suppressions, and adding inference-only annotations whose primary value was the VerifyTypes score.

Commit 116f4c9 restores tomlkit/container.py and tomlkit/items.py exactly to current master, removing every suppression and annotation introduced by this PR. No independently valuable, checker-clean residual change remained, so the PR no longer has a meaningful diff.

Cleanup validation:

• poetry run pre-commit run --all-files (pass)
• poetry run pytest -q tests (1,051 passed)
• poetry run mypy (same 29 pre-existing clean-master errors)
• poetry run pyright --verifytypes tomlkit --ignoreexternal (restored master baseline: 382 known / 47 ambiguous / 44 unknown across 473 exports; 80.7611%)
• runtime API/signature/concrete-base and annotation-neutral AST comparison (exact baseline match)
• focused collection, copy, pickle, operator, and Python 3.9 checks (pass)

Agent Drafting Metadata

• Agent: GitHub Copilot CLI
• Model: GPT-5.6 Sol
• Notes: The agent performed the policy cleanup, validation, and closure under explicit user authorization.

[heejaechang]

Closing after policy cleanup: the branch now matches master exactly, and no independently valuable checker-clean residual change remains.