Reject unreachable partial varargs after keyword binding

[xiehuanyi]

partial(f, first=1) still advertises *args when f has variadic positional parameters, even though a positional call raises TypeError for the duplicate first argument. Remove that unreachable variadic parameter while preserving the positional prefix and legal keyword-only arguments. This completes the variadic case of #2191 after #2446.

Validation: 69 curry type cases pass, including four new regressions/controls. The complete runtime and documentation suite passes (1112 passed, four expected failures, 100% coverage). Full Ruff lint/format, Flake8, Mypy, slots checks and package compatibility pass on Python 3.15.

• No unrelated changes
• Regression type tests added
• Changelog updated

Prepared with OpenAI Codex (GPT-6.1 Sol, xhigh), including this description.

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[codspeed]

Merging this PR will not alter performance

ℹ️ 2 benchmarks have no flame graph

The flamegraph could not be generated because we failed to resolve the symbols in the profiling trace. This happens when we fail to harvest information about language frames.

• test_fold_collect_results
• test_maybe_do_notation

✅ 22 untouched benchmarks

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_{Comparing xiehuanyi:issue-2191-variadic-20261012-r2 (166d0b7) with master (6313ed7)}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (82ef3ef) to head (166d0b7).
⚠️ Report is 633 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##            master     #2537    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           80        82     +2     
  Lines         2485      2692   +207     
  Branches       437        44   -393     
==========================================
+ Hits          2485      2692   +207     

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