test(mv3): prove ephemeral Agent Task profile isolation

[seonghobae]

Buyer-visible gap

Partial implementation of #28. The pinned real-Chromium compatibility lane must prove that every Agent Task trial starts from an empty profile, reuses only that trial-local profile across the controlled restart pair, removes it afterwards, and does not leave inherited Chromium descendants alive after ChromeDriver exits.

Fresh live stack

Current exact prerequisite PR #43 head is 28e79febd47361c5fd41e45db7e1429b179afdd1 (test/mv3-downloads). Current exact child head is 55ddddc26f309323d717684540236e1fa768edbc (test/mv3-ephemeral-profile-isolation).

Fresh exact comparison #43 → #49 reports ahead 37 / behind 0, with merge base exactly the live prerequisite. The child-owned semantic delta is exactly three files:

• scripts/ci/run_mv3_compatibility.py;
• tests/test_mv3_ephemeral_profile_contract.py; and
• tests/test_mv3_process_group_cleanup_contract.py.

The child therefore descends cleanly from the current prerequisite after parent movement. The previously documented prerequisite/head pair ed15185a... / 90849dfb... is predecessor narrative only and is not promoted as current evidence. No prerequisite/predecessor check or review transfers.

Implemented boundary

The current head preserves the prerequisite's downloads/status/executable/diagnostic/cleanup and bounded ChromeDriver-startup-output hardening and adds:

• one fresh temporary Chromium profile per compatibility trial;
• reuse of only that trial-local profile across the controlled initial/restart pair;
• profile deletion after the pair, with the profile path absent from returned evidence;
• ChromeDriver launch in a fresh operating-system session;
• process-group SIGTERM with bounded SIGKILL recovery so inherited Chromium descendants do not retain the supposedly ephemeral profile;
• bounded fallback behavior for already-gone or failing process-group signals without catch-all suppression;
• causal error preservation: a primary browser-pass failure remains primary if session cleanup/process teardown also fail, while a reviewed session-delete failure remains the cause of typed WebDriverSessionCleanupError when teardown also fails; and
• inherited prerequisite ChromeDriver startup authority: malformed status shapes remain bounded protocol failures, a Ready endpoint must identify the pinned ChromeDriver build, and startup records are consumed with bounded reads.

The process-group behavior is controlled Linux CI evidence, not a claim of a complete cross-platform production process supervisor.

Exact-current verification

On unchanged exact child head 55ddddc26f309323d717684540236e1fa768edbc against unchanged exact prerequisite 28e79febd47361c5fd41e45db7e1429b179afdd1 at the latest refetch:

• ancestry: ahead 37 / behind 0, merge base exactly the live prerequisite;
• semantic child delta: exactly the three files listed above;
• CI run 33201171274: success;
• Manifest V3 Compatibility run 33201171264: success;
• formal reviews currently returned: none;
• inline review threads currently returned: none; and
• GitHub reports the PR Draft and mergeable.

Feature-base organization security/review workflows not executed on this exact head are absent evidence, not passing. No prerequisite, predecessor, queued, skipped, cancelled, synthetic, status-only, or model-only result is promoted as current proof.

Scope / governance

This remains compatibility evidence, not a production browser adapter. It adds no default-profile access, raw credential handling, persistent profile storage, extension grant widening, general browser-protocol authority, model call, workflow mutation, or release claim. Keep Draft while #43 remains active. Protected-main AGENTS.md forbids this scheduled actor from merging, self-approving, altering workflows, adding secrets, weakening checks, tagging, or publishing.

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