fix: eliminate comply tester false positive observations

[sliss]

Summary

• valid_actions / sandbox false positives: Skip snapshot-only get_media_buys previews (which lack a status field) when checking for valid_actions and sandbox in the observation builder. Snapshot previews intentionally omit these fields, causing false warnings.
• canceled_by false positive: Handle nested media_buy response envelope when extracting canceled_by, canceled_at, and revision from the cancel step in testMediaBuyLifecycle, matching how extractStatus already handles nesting.
• Controller validation stderr noise: Suppress Schema validation failed for comply_test_controller console output during callController calls, since the controller intentionally returns non-standard error codes (UNKNOWN_SCENARIO, INVALID_PARAMS, NOT_FOUND) that fail schema validation.

Test plan

• Run adcp comply against OpenAds test server and confirm no false positive advisories for valid_actions, sandbox, or canceled_by
• Confirm no noisy stderr from expected controller error responses
• Verify all existing scenarios still pass (no regressions)

🤖 Generated with Claude Code

[github-actions]

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