ci(python): harden uv supply chain checks

[EfeDurmaz16]

Summary

This is a small Python CI supply-chain hardening follow-up targeting split/pr216-ci-harness-mega (#219).

• Enable UV_MALWARE_CHECK=1 with explicit audit-command,malware-check preview opt-in for Python test, harness, and PyPI publish jobs.
• Pin every astral-sh/setup-uv runtime used by these workflows to 0.11.26.
• Add blocking uv audit --frozen checks to the Python test, standalone harness, and privileged publish paths.
• Update the lockfile for the audited Django, pydantic-settings, Starlette, and Black advisories.
• Pin the publish build backend and release tools in the lockfile, build without isolation, and run Twine from the frozen environment instead of floating uvx twine.
• Add a structural guard with negative fixtures for runtime pins, job-level malware policy, frozen syncs, and unconditional audits.
• Route changes to the guard through the Python workflow selector.

The Black override is narrowly required because docspec-python 2.2.1 still declares a stale Black <24 constraint. The docs command is run from the frozen environment to exercise that compatibility path.

Verification

• uv lock --check
• uv audit --frozen: no known vulnerabilities or adverse project statuses in 120 audited packages
• Malware-gated frozen syncs for dev, framework, and release environments
• Python: 1308 passed, 1 skipped, 91.73% coverage
• Ruff, Pyright, frozen docs smoke, release build, and strict Twine check
• Workflow YAML parsing, selector tests, repo hygiene, publish guard, and Python supply-chain guard negative fixtures

CI dependency

The full shared matrix currently inherits the known conformance_runner failure on #219. That runner belongs to the Rust delivery in #227 and is intentionally not duplicated in this Python CI PR. The exact-head matrix should be rerun after that base fix lands.

This PR is intentionally based on #219 and does not change payment protocol behavior.