fix: anchor PAC paths to the trusted task cache

[Gene Chu (genec314)]

Summary

• Replace substring-based PAC path validation with validation anchored to the executing task bundle's task cache.
• Require exact installer/version/bin layout, validate canonical filesystem containment and executable shape, and cache the canonical runner directory.
• Add native filesystem regression fixtures for repository-local imitations, invalid path shapes, links, and directory identity.

Tasks affected

The shared runner parameters are used by 32 PAC-consuming v2 tasks, including credential-free pack/unpack tasks. Installer import and credential-only task behavior remain unchanged.

Architecture impact

No task inputs, service-connection schemas, authentication flows, or dependencies are changed. Validation remains lazy; the runner directory getter now retains the validated canonical path. Existing Windows/Linux platform layouts are retained.

Known limitations / follow-ups

This is a draft, not release-ready sign-off.

• Dependencies could not be restored because the locked package-feed download timed out and direct public npm access failed TLS negotiation. The locked CLI wrapper implementation remains unavailable for executable-selection and pre-spawn contract inspection.
• Default deployed-location/getter, canonical caching, runner-boundary, and emitted-bundle coverage still need completion.
• Native Windows case-sensitive installer/bin sibling and drive/share-alias coverage remains incomplete.
• The task cache, executable, and support files must be protected from attacker modification. Path validation is not protection against same-user file replacement or check/use races.

Test plan

• Focused static source review; no high-confidence defect identified within the documented protected-cache trust model.
• Execute the focused path/getter/runner/bundle regressions.
• Run the existing unit suite, lint, TypeScript check, and build.
• Inspect the locked wrapper and emitted/packaged runtime layout.
• Run an authorized DEV-stage pipeline and complete native Windows/Linux coverage.

No tests, builds, or PAC were executed locally; execution-based validation was explicitly deferred.

[Gene Chu (genec314)]

@microsoft-github-policy-service agree company="Microsoft"