fix: handle case-insensitive retrieval in GoogleSecretManagerSettingsSource

[ezwiefel]

Fixes #728.

This PR updates GoogleSecretManagerMapping to support robust case-insensitive secret retrieval.

Changes:

• Maintains a map of both exact and lowercased secret names to their actual GCP identifiers.
• Ensures that secrets with uppercase or mixed-case names (e.g. TEST_SECRET, mySecret) are correctly retrieved when case_sensitive=False.
• Implements deterministic collision resolution: if multiple secrets normalize to the same lowercase key (e.g. MY_SECRET and my_secret), the lexicographically last one (preferring lowercase) is chosen.
• Issues a UserWarning when such collisions occur.
• Adds regression tests covering various casing scenarios and collision handling.
• Updates documentation to explain the new behavior.

[hramezani]

Thanks @ezwiefel