feat(functions): Add FunctionScope and update task_queue to support kit scopes

[inlined]

Explanation

This PR introduces support for explicit task queue scopes to match the design patterns introduced in firebase-admin-node#3210.

It adds the new FunctionScope class and updates the task_queue signature to accept a scope parameter (while deprecating extension_id). It also implements an automatic fallback retry flow that shifts queue targets from extensions to kits upon receiving a 404 Not Found response.

To ensure complete thread safety without requiring locks, the TaskQueue class is stateless/immutable, delegating requests to private helper functions _enqueue_with_scope and _delete_with_scope that accept the resolved scope as a parameter.

Key Changes:

• FunctionScope: Introduced a validation-enforced scope type representing 'current', 'global', 'extension', and 'kit'.
• task_queue update: Added the scope parameter, deprecated extension_id, and bound warnings appropriately.
• Stateless Fallback Flow: Refactored enqueue and delete methods to use private helper functions. If a hybrid scope ('extension_or_kit') hits a 404, it retries targeting the kit scope. If the fallback kit queue deletes fail with 404, the NotFoundError exception is correctly propagated back to the user (preserving original Python SDK behavior).
• Environment Variable Rename & Support: Renamed environment variable KIT_INSTANCE_ID to FIREBASE_KIT_INSTANCE_ID and added support for EXT_INSTANCE_ID in current scope (with extension taking precedence).
• Type Compatibility: Added Tuple import from typing and updated signatures to support Python versions older than 3.9 (e.g. 3.7 and 3.8).

Testing Strategy

• Modified existing unit tests to match the dynamic scope architecture.
• Added new unit test suite (TestFunctionScope) covering constructor validations and bounds.
• Added unit tests for environmental configurations (FIREBASE_KIT_INSTANCE_ID, EXT_INSTANCE_ID and precedence).
• Added robust fallback retry tests using MockMultiRequestAdapter verifying that 404 errors triggers fallbacks, that successful retries log user warnings, and that failures propagate properly.
• Project-wide linter rate is verified at 10.00/10.
• Checked and verified that all 3908 unit tests pass successfully.

[gemini-code-assist]

Code Review

This pull request introduces the FunctionScope class to define function scopes in task queues, replacing the deprecated extension_id parameter in task_queue with a new scope parameter. It also implements fallback logic to retry with 'kit' scope upon receiving a 404 error for 'extension_or_kit' scopes, and adds comprehensive unit tests. The feedback suggests refactoring the error handling in the delete method to use a guard-clause style for consistency with the enqueue method.