PR summary

closes #31702

This PR fixes a bug where using ax.clabel with the manual keyword raises an IndexError if the user specifies a subset of contour levels (fewer than the total available in the contour set).

What problem does it solve and reasoning:
In lib/matplotlib/contour.py, the add_label_near function was incorrectly using idx_level_min to access self.labelLevelList and self.labelCValueList. Because idx_level_min corresponds to the index of the entire contour set, passing a filtered subset of levels caused it to search out-of-bounds.

As suggested by the original issue author, I swapped idx_level_min for the level variable, which correctly maps to the current subset being labeled. I also added a minimal pytest in test_contour.py that verifies a manual label can be placed on a subset contour without throwing an exception.

AI Disclosure

I used an AI assistant strictly to help navigate the repository structure, locate the specific lines in contour.py, and draft the boilerplate for the minimal pytest function. The core logic fix was suggested by the issue author, and I manually applied, reviewed, and tested the changes locally to ensure complete Matplotlib compliance.

PR checklist

• "closes [Bug]: manual positioning fails for ax.clabel #31702" is in the body of the PR description to link the related issue
• new and changed code is tested
• [N/A] Plotting related features are demonstrated in an example
• [N/A] New Features and API Changes are noted with a directive and release note
• [N/A] Documentation complies with general and docstring guidelines