Reject malformed parent secret responses without panicking

[AnthonyRonning]

Summary

• isolate parsing of secret responses received from the Nitro parent
• return normal errors for malformed JSON, non-string payloads, and invalid secret shapes instead of panicking or silently returning an empty value
• require exactly one string-valued field in the decoded Secrets Manager object
• cover valid, truncated, malformed, unexpected-type, non-string, empty, and multi-field responses with deterministic tests

Security boundary

This response is read from the internal Nitro parent over vsock CID 3 / port 8003. The existing read path is capped at 256 KiB and uses 30-second I/O timeouts, and the audit found no public or customer-controlled route into this parser. This PR hardens that internal trust boundary without changing the wire format.

Validation

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test parent_secret_response_tests -- --nocapture (6 passed)
• cargo test --all-targets --all-features (219 passed, 16 existing database-dependent tests ignored)

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Review details

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: bad70f25-244d-4283-8c2b-184c0e24bdd2

📥 Commits

Reviewing files that changed from the base of the PR and between a3ecc93 and 3f70363.

📒 Files selected for processing (2)

• src/main.rs
• src/parent_secret_response_tests.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex-opensecret-parent-secret-parser-opensecret

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 1 additional finding.