Isolate host Python imports in the pinned PHP CI setup #342

Delivered in merged #343 at signed main 799df847137dafd3fb70968c94ae56609a313479. Its complete tree 8f45464a88417e6e5619e4b15149cef02a57439e equals Security-reviewed source f847a62d8cc483460bdf0bd21e7468196a2ce137.

Both host Python entry points now use -I -S -B. The TLS control executes directly so unittest discovery cannot add its checkout directory to the import path. Four marker regressions fail the predecessor and pass the fix, all 77 offline checks pass with one declared runtime-only skip, and actual PHP 8.4/8.5 Composer TLS accept/refuse-before-HTTP controls pass. Exact source/runtime evidence, Security approval.

The existing narrow runtime record is accepted through central383, signed main 03438ccdd761e6c821bbd8f36f939982e3065115. Independent actual-consumer validation, missing-verifier/changed-setup refusals and 75 focused trust controls passed. All 30 current-head PR checks then passed after rerunning only the failed policy/aggregate jobs.

Actual-main acceptance is complete for every admitted caller and protected check:

• CI43426250847: PHP 8.4/8.5 application and microservice cells, central preflight, image-security controls and required aggregate all pass.
• Playground43426251239: the real PHP/Python/Rust journeys pass, completing the fifth caller.
• Lock audit43426316684: sixth caller passes on this exact main SHA. This existing workflow was dispatched because its paths omit setup-helper changes.
• Smoke43426250890 and polyglot43426251607: both remaining protected checks pass.

The merged remote topic is deleted and a direct GitHub lookup verifies HTTP404. The local source worktree remains available; no Sample App containers remain. The three unchanged SDK timer experiment runs 43426250694, 43426250479 and 43426251166 have subsequently passed. All 16 actual-main workflows are now terminal success. The corrected product-tuple qualification remains separate work in #338.

Owner: sample-app. All acceptance criteria for this bounded host-Python correction are complete. The protected developer-image and current product tuple follow-through remains in #338/#339; shared #297 remains open for its other caller profiles.