Add QuotaFailure to the FirebaseError

[justinattw]

Based on (and requires first merging of) #737
Closes #736

https://firebase.google.com/docs/reference/fcm/rest/v1/ErrorCode specifies that google.rpc.QuotaFailure is returned with FCM.

We can expose this via a public API GetQuotaFailure(err *errorutils.FirebaseError) *QuotaFailure method.

I also refactored the messaging.go error-related code bits into messaging_errors.go.

[gemini-code-assist]

Summary of Changes

Hello @justinattw, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly refactors and enhances error handling within the Firebase Admin Go SDK. It introduces a structured way to parse and expose detailed quota failure information from FCM responses, centralizes common error types and utilities into a new public package, and improves the internal organization of the messaging client's error logic. These changes aim to provide developers with more granular and actionable error insights while maintaining an idiomatic Go experience.

Highlights

• Enhanced FCM Error Reporting: Introduced a new QuotaFailure type and GetQuotaFailure function in the messaging package to provide detailed information about google.rpc.QuotaFailure errors returned by FCM, such as specific quota violations.
• Public Error Utilities Package: Refactored the core FirebaseError type, ErrorCode constants, and common error-checking utility functions into a new, public errorutils package, improving modularity and reusability across the SDK.
• Messaging Error Code Refactoring: Moved all FCM-specific error codes and error handling logic from messaging.go to a new dedicated file, messaging_errors.go, enhancing code organization within the messaging package.
• Copyright Updates: Updated copyright notices across numerous files from 'Google Inc.' to 'Google LLC' for consistency.
• AI Agent Guide Added: A new AGENTS.md file has been added, providing comprehensive guidelines for AI agents contributing to the Firebase Admin Go SDK, covering conventions, design patterns, and architecture.
• FirebaseError Field Renamed: The String field within the FirebaseError struct has been renamed to Message for improved clarity and consistency in error reporting.

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Ignored Files

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[gemini-code-assist]

Code Review

This pull request introduces support for QuotaFailure errors in FCM and significantly refactors the error handling logic across the SDK. The core error types and helper functions have been moved from an internal package to a new public errorutils package, which is a great improvement for SDK users. The new QuotaFailure parsing logic for FCM errors is well-implemented and comes with thorough tests.

Overall, the changes are well-structured and improve the SDK's error handling capabilities. I've only found a couple of minor typos in the newly added AGENTS.md documentation file.