Actions: Normalize explicit untrusted checkout paths

[1sgtpepper]

Fixes #22213.

Summary

• Canonicalize explicit relative actions/checkout paths and static local script or action operands before component-aware containment checks.
• Preserve the existing fallback for dynamic checkout paths and workspace-root execution.
• Add focused pull_request_target and workflow_run coverage for bare, dot-relative, trailing-slash, nested, quoted, sibling, and escaping paths.

Workspace-variable and context-expression resolution remain out of scope. This PR does not change shared path normalization or poisonable-step modeling.

Validation

• codeql test run actions/ql/test/query-tests/Security/CWE-829 actions/ql/test/query-tests/Security/CWE-349 actions/ql/test/library-tests/basic --threads=1 (15/15 passed)
• codeql query format --check-only for the changed QL files
• git diff --check

The expected query output was generated with codeql test run --learn, inspected for the intended Critical results, and verified with a non-learning run.

[1sgtpepper]

@oscarsj, the current draft fixes #22213. I’m apprehensive about expanding this PR into shared path-model behavior, so I think the broader cases should be deferred to a separate follow-up. Does that scope sound right?