feat(core): Error Handling Revamp

[jonathanedey]

This PR makes changes the usage and handling of Firebase Errors and includes the following changes:

• Exposes all service-specific error classes, error code types and base FirebaseError class.
• Exposes the associated uniform error codes types for those respective errors.
• Decouples previous client-facing error codes from internal SDK details and refactor error handling into dedicated, modular error.ts files, utilizing declaration merging to improve IDE autocomplete and indexing.
• Exposes ErrorInfo and HttpResponse classes for better error handing.
• Adds 2 new fields to ErrorInfo FirebaseErrors: cause and HttpResponse.
• Refactors how FCM HTTP2 session errors are handled making use of the new cause attribute on FirebaseErrors to allow failing of request at the stream level with context on session error cause.
• Removes internal FirebaseMessagingSessionError and its logic.

[gemini-code-assist]

Code Review

This pull request refactors the SDK's error handling system by introducing a standardized ErrorInfo interface that includes HTTP response data and underlying error causes. Service-specific error classes now extend PrefixedFirebaseError and utilize object-based constructors. The messaging service's HTTP/2 logic was also updated to aggregate session errors into batch responses. Feedback identifies opportunities to improve AggregateError serialization in toJSON(), remove the obsolete FirebaseMessagingSessionError class, and refine error message formatting within the HTTP/2 session handler.

[gemini-code-assist]

Since AggregateError is now being used as a cause (specifically in the messaging service), the toJSON() method should be updated to serialize the nested errors array. Without this, critical debugging information about multiple failures will be lost when the error is serialized.

    if (this.cause) {
      json.cause = {
        name: this.cause.name || 'Error',
        message: this.cause.message || String(this.cause),
        stack: this.cause.stack
      };
      if ('errors' in this.cause && Array.isArray((this.cause as any).errors)) {
        (json.cause as any).errors = (this.cause as any).errors.map((e: any) => {
          if (e instanceof Error) {
            return { name: e.name, message: e.message, stack: e.stack };
          }
          return String(e);
        });
      }
    }

[gemini-code-assist]

The FirebaseMessagingSessionError class appears to be dead code. The PR description mentions that it and its logic were removed, and the refactoring in src/messaging/messaging.ts confirms that it is no longer thrown. It should be removed from this file as well.

[gemini-code-assist]

The error message construction in the error event listener can be improved for better readability and type safety. The errorMessage variable should be typed as string, and the logic can be simplified to avoid potential undefined strings and unnecessary trailing spaces.

Suggested change

	this.sessionErrors.push(error);
	});
	http2Session.on('error', (error: any) => {
	this.http2Session.on('error', (error: any) => {
	let errorMessage: any;
	if (error.name == 'AggregateError' && error.errors) {
	errorMessage = `Session error while making requests: ${error.code} - ${error.name}: ` +
	`[${error.errors.map((e: any) => e.message).join(', ')}]`
	if (error?.name === 'AggregateError' && Array.isArray(error.errors)) {
	errorMessage = `Session error while making requests: ${error?.code} - ${error?.name}: ` +
	`[${error.errors.map((e: any) => e.message).join(', ')}]`;
	} else {
	errorMessage = `Session error while making requests: ${error.code} - ${error.message} `
	errorMessage = `Session error while making requests: ${error?.code} - ${error?.message} `;
	}
	this.http2Session.on('error', (error: any) => {
	const codePart = error?.code ? `${error.code} - ` : '';
	let errorMessage: string;
	if (error?.name === 'AggregateError' && Array.isArray(error.errors)) {
	errorMessage = `Session error while making requests: ${codePart}${error.name}: ` +
	`[${error.errors.map((e: any) => e.message).join(', ')}]`;
	} else {
	errorMessage = `Session error while making requests: ${codePart}${error?.message || 'Unknown error'}`;
	}
	const appError = new FirebaseAppError({
	code: AppErrorCode.NETWORK_ERROR,
	message: errorMessage,
	cause: error,
	});
	this.sessionErrors.push(appError);
	});