test(review): cover empty Mistral key CLI failure

[pierrick-fonquerne]

What changed

Adds a local integration test that runs the review executable with a blank Mistral key and asserts the explicit failure contract.

Why

Prevents a regression where an invalid review configuration could be accepted by the executable path.

Validation

• cargo test --manifest-path tools/ai-review/Cargo.toml --test empty_mistral_key
• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --workspace --all-features --no-fail-fast
• cargo build --release