feat: accept Expression for v2 RTDB instance option

[IzaakGough]

Fixes #1613

Summary

Allow the v2 Realtime Database trigger instance option to accept a deploy-time param (Expression<string>), not just a plain string.
This lets functions target different RTDB instances per environment without triggering the deployment-time .value() warning.

Problem

ReferenceOptions.instance was typed and handled as a plain string.

Passing defineString('REALTIME_INSTANCE') for instance either failed type-checking or caused the SDK to eagerly resolve the param during deploy, which emitted the standard .value() deployment warning.

Solution

• Widen ReferenceOptions.instance to string | Expression<string>.
• Preserve expression-backed instances through endpoint generation instead of resolving them in getOpts().
• Serialize expression-backed instances into the manifest as CEL expressions.
• Resolve the expression only at runtime when the trigger needs to construct its path pattern and event params.

Testing

• Added unit coverage for getOpts() preserving an Expression<string>.
• Added endpoint/manifest coverage for expression-backed instance values.
• Added a test that verifies no deployment-time warning is emitted when defining the trigger with a param-backed instance.
• Added a runtime test confirming instance resolution and param extraction still work.

[gemini-code-assist]

Code Review

This pull request enhances Realtime Database (RTDB) ReferenceOptions to allow Expression<string> for the instance property, enabling dynamic resolution of instance names. The getOpts utility function has been updated to process these expressions, and a new test case validates this functionality. The review suggests improving the new test case by replacing magic strings with named constants for better readability and maintainability.

[CorieW]

Does this not cause a warning log during deployment?

[IzaakGough]

Yes you're right, thanks for spotting this. I've changed the RTDB trigger path to preserve Expression through endpoint generation and only resolve it at runtime, so the warning no longer appears during deploy.

All tests pass and I did a manual deployment test with a dummy function, seems to work fine.