Detect Google API keys in AG001

[amic25]

Goal

Extend AG001 with a high-confidence Google API key signature.

Scope

• Add the pattern to HardcodedSecretRule.
• Add synthetic positive and negative tests.
• Ensure common documentation placeholders do not trigger.
• Never use a real credential in fixtures or discussion.

Acceptance criteria

• A realistic synthetic key is detected at the correct line and column.
• Placeholder/example values remain clean.
• Existing secret rules and make check pass.