🛡️ Sentinel: [HIGH] Fix authorization bypass in JWT validation

[mudcube]

🚨 Severity: HIGH
💡 Vulnerability: The validate_token_with_project function in mill-auth failed open when the project_id claim was missing from the JWT token, returning success to maintain backward compatibility. This could allow tokens without project scoping to bypass project-specific authorization checks.
🎯 Impact: An attacker with a valid but unscoped JWT could potentially access project-specific resources they should not have access to, bypassing multi-tenant isolation.
🔧 Fix: Updated validate_token_with_project to fail closed by explicitly returning a MillError::permission_denied error if the expected project_id claim is missing from the token. Added explicit security comments explaining the change.
✅ Verification: Run cargo test -p mill-auth to ensure test_validate_token_with_project_no_claim now asserts that validation fails when the claim is missing. Tested with cargo clippy and cargo fmt.

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PR created automatically by Jules for task 8504323169223189950 started by @mudcube

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[chatgpt-codex-connector]

Enforce project claims on the websocket auth path

This fail-closed branch does not protect the production websocket authentication path: rg validate_token_with_project shows no production callers, while crates/mill-transport/src/ws.rs:208-220 still decodes JWTs directly and returns Ok(response) after only warning when project_id is absent; handle_initialize then accepts the client-supplied project at ws.rs:459-460. In deployments using websocket auth, a valid unscoped token can therefore still connect and choose any project despite this helper rejecting the same token in tests.

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