Fix PolarAxes.format() silently dropping xlabel/ylabel

[kinyatoride]

Summary

ax.format(xlabel=..., ylabel=...) had no effect on polar axes (e.g. for Taylor-diagram-style figures), even though ax.set_xlabel(...) worked. PolarAxes.format() did not declare xlabel/ylabel parameters and base.Axes.format() does not handle them — only CartesianAxes.format() did.

This PR:

• Adds an _update_labels helper to PolarAxes mirroring the CartesianAxes one, and wires xlabel, ylabel, xlabel_kw, ylabel_kw through the existing (x, y) loop so they are forwarded to matplotlib's set_xlabel/set_ylabel.
• Fixes a dispatcher limitation in Figure.format() exposed by the change: _pop_params previously popped each kwarg into the first matching class signature, so when called via SubplotGrid.format → Figure.format on a polar axes, xlabel was routed to kws[CartesianAxes] and never applied. Switched to a non-popping collector so shared params reach every matching class signature; the unused-warning is updated so shared keys consumed by any matched class don't trigger a false warning.
• Adds a regression test in test_projections.py.

Test plan

• pytest ultraplot/tests/test_projections.py -k polar — new and existing polar tests pass
• pytest ultraplot/tests/test_format.py — no regressions
• Full suite: 523 passed (1 unrelated failure in test_docs_fetch_releases)
• Smoke test: ax.format(xlabel='Radius', ylabel='Angle', labelpad=15, labelcolor='red', xlabel_kw={'weight': 'bold'}) renders correctly with per-axis-kw override

[cvanelteren]

Out of town atm can look at this when I'm back.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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