Add protocol compliance testing helpers

[FlorianPfaff]

Summary

Adds reusable protocol-compliance testing helpers:

• pyrecest.protocols.testing
• ProtocolAssertionError
• generic helpers for runtime protocol checks, attributes, callable methods, non-None method results, and array shape checks
• convenience helpers for common PyRecEst capabilities: dimensions, distribution-like methods, minimal filter contract, likelihood models, and transition models
• focused unit tests for all helpers
• docs section showing how to use the helpers in capability tests

Scope

This is intentionally additive and independent of the distribution/filter/model/conversion/manifold protocol PRs. It does not change existing distribution, filter, model, conversion, or manifold behavior.

The helpers use duck typing plus the common protocols from PR0 so they can be used immediately and refined later as more protocol modules land.

Non-goals

Not included:

• new distribution/filter/model protocols
• package-level exports from pyrecest.protocols
• py.typed
• changes to existing abstract base classes
• mathematical correctness tests for any concrete estimator or distribution

Tests

Focused local checks:

PYTHONPATH=src python -m compileall -q src/pyrecest/protocols tests/protocols/test_protocol_testing_helpers.py
PYTHONPATH=src pytest -q tests/protocols/test_protocol_testing_helpers.py

Result:

12 passed

Stacking

This PR is stacked on #1952 and targets feature/public-protocol-seed. After #1952 is merged, this PR can be retargeted to main if needed.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	17.62s
✅ JSON	prettier	2	0	0	0	0.52s
✅ JSON	v8r	2		0	0	3.12s
✅ MARKDOWN	markdownlint	28	0	0	0	1.39s
✅ MARKDOWN	markdown-table-formatter	28	1	0	0	0.34s
✅ PYTHON	bandit	478		0	0	7.52s
✅ PYTHON	black	478	22	0	0	13.49s
✅ PYTHON	flake8	478		0	0	4.18s
✅ PYTHON	isort	478	27	0	0	0.91s
✅ PYTHON	mypy	478		0	0	6.3s
✅ PYTHON	pylint	478		0	0	108.92s
✅ PYTHON	ruff	478	27	0	0	0.11s
✅ REPOSITORY	checkov	yes		no	no	23.19s
✅ REPOSITORY	gitleaks	yes		no	no	13.31s
✅ REPOSITORY	git_diff	yes		no	no	0.08s
✅ REPOSITORY	secretlint	yes		no	no	8.99s
✅ REPOSITORY	syft	yes		no	no	3.42s
✅ REPOSITORY	trivy-sbom	yes		no	no	3.63s
✅ REPOSITORY	trufflehog	yes		no	no	21.54s
✅ YAML	prettier	5	0	0	0	0.71s
✅ YAML	v8r	5		0	0	5.74s
✅ YAML	yamllint	5		0	0	0.5s

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[github-actions]

Test Results

    12 files  ±  0      12 suites  ±0   4h 42m 21s ⏱️ + 26m 15s
 1 375 tests + 12   1 375 ✅ + 12      0 💤 ±0  0 ❌ ±0 
16 864 runs  +144  13 288 ✅ +144  3 576 💤 ±0  0 ❌ ±0 

Results for commit 90f7a54. ± Comparison against base commit f9cd9d3.

♻️ This comment has been updated with latest results.