timedelta: support microseconds and negative values

[methane]

ref: PyMySQL/PyMySQL#1262

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.66%. Comparing base (e2a908b) to head (2487dee).
⚠️ Report is 49 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #795      +/-   ##
==========================================
+ Coverage   88.86%   90.66%   +1.79%     
==========================================
  Files           7        7              
  Lines         548      557       +9     
==========================================
+ Hits          487      505      +18     
+ Misses         61       52       -9     

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[Copilot]

Pull request overview

This PR updates MySQLdb’s timedelta string formatting to correctly handle negative values and to include microseconds, aligning both literal conversion and formatting behavior with MySQL’s expected TIME literal semantics.

Changes:

• Fix format_TIMEDELTA to render negative timedeltas with a single leading sign and an absolute-value day/time component (avoids MySQL interpreting a “double-negative”).
• Extend format_TIMEDELTA output to include fractional seconds when timedelta.microseconds is non-zero.
• Expand/adjust tests to cover negative timedeltas and microsecond precision, and update an existing negative-case expectation.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
tests/test_MySQLdb_times.py	Adds/updates assertions for negative and microsecond-precision timedelta formatting and literals.
src/MySQLdb/times.py	Revises format_TIMEDELTA to correctly handle negative timedeltas and microseconds.

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