Adapt decoder calibration CV for sparse folds

[FlorianPfaff]

Summary

• Add an adaptive calibration wrapper for decoder factories that currently request CalibratedClassifierCV(cv=3).
• Reduce the inner calibration CV at fit time to the largest feasible value given the fitted fold's per-class counts.
• Fall back to uncalibrated score-derived probabilities only when a calibration split is impossible, preserving probability-shaped emissions for sparse smoke tests and small held-out-subject folds.
• Add regression tests for sparse calibrated linear-SVM folds.

Testing

• Added tests/test_adaptive_calibration.py for the 2-per-class adaptive CV case and the 1-per-class fallback case.
• I sanity-checked the wrapper behavior locally with installed NumPy/scikit-learn. I could not run the full repository test suite locally because the execution container cannot clone GitHub repositories directly, so CI should be treated as the authoritative validation.

[github-actions]

Test Results

    4 files  ±0      4 suites  ±0   1m 47s ⏱️ +5s
  641 tests +2    632 ✅ +2   9 💤 ±0  0 ❌ ±0 
2 564 runs  +8  2 528 ✅ +8  36 💤 ±0  0 ❌ ±0 

Results for commit c80d67e. ± Comparison against base commit 46ac769.

[github-actions]

✅⚠️MegaLinter analysis: Success with warnings

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ ACTION	actionlint	14		0	0	0.46s
✅ COPYPASTE	jscpd	yes		no	no	8.65s
⚠️ MARKDOWN	markdownlint	55	0	1	0	1.78s
✅ PYTHON	ruff	245	0	0	0	0.39s
✅ REPOSITORY	git_diff	yes		no	no	0.06s
✅ YAML	prettier	48	0	0	0	1.48s
✅ YAML	v8r	48		0	0	8.23s
✅ YAML	yamllint	48		0	0	1.3s

Detailed Issues

⚠️ MARKDOWN / markdownlint - 1 error

.github/PULL_REQUEST_TEMPLATE.md:1 error MD041/first-line-heading/first-line-h1 First line in a file should be a top-level heading [Context: "## Summary"]

Notices

📣 MegaLinter 9.5.0 is out! Discover the new features and security recommendations in the release announcement. (Skip this info by defining SECURITY_SUGGESTIONS: false)

See detailed reports in MegaLinter artifacts

Show us your support by starring ⭐ the repository