fix(fcm): Map topic subscription RESOURCE_EXHAUSTED server code to new TOPICS_SUBSCRIPTION_RATE_EXCEEDED SDK error code

[jonathanedey]

Previously we incorrectly mapped the topic subscription RESOURCE_EXHAUSTED FCM server code to TOO_MANY_TOPICS SDK error code leading to misleading error messaging for the cause of the error. This PR introduces a new TOPICS_SUBSCRIPTION_RATE_EXCEEDED to correctly attribute the error.

RELEASE_NOTE: Mapped the topic management RESOURCE_EXHAUSTED server error to a new TOPICS_SUBSCRIPTION_RATE_EXCEEDED code instead of TOO_MANY_TOPICS to improve subscription rate limits error messaging.

DO_NOT_MERGE: This PR should be merged into the v14 release branch directly. Temporally staging against error-revamp until those changes are merged into v14 separately.

[gemini-code-assist]

Code Review

This pull request introduces a new error code, TOPICS_SUBSCRIPTION_RATE_EXCEEDED, to handle rate-limiting scenarios in topic management. It updates the server-to-client error mapping so that RESOURCE_EXHAUSTED now correctly maps to this new code instead of TOO_MANY_TOPICS. Feedback was provided to generalize the error message to cover both subscription and unsubscription operations, as the current wording is specific to subscribing.