chore: improve tomli maintenance path

[lphuc2250gma]

Summary:

• Add focused unit tests covering specific TOMLDecodeError edge cases (e.g., precise line/column reporting for malformed inline tables, unterminated multi-line strings, or invalid datetime offsets) — verifying the exception message and pos attribute, not changing parser behavior.
• Keep the change narrow so it is straightforward to review.

Notes:

• I kept this scoped to the relevant implementation and tests.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (5a77b12) to head (9f23cb1).

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #300   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         4           
  Lines          535       535           
  Branches       100       100           
=========================================
  Hits           535       535           

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