WIP feat(fcm): Migrate topic subscription APIs to new REST endpoints

[lahirumaramba]

Migrate topic subscription APIs to new REST endpoints via HTTP/2 multiplexing

DO_NOT_MERGE

[gemini-code-assist]

Code Review

This pull request refactors topic management to use the FCM v1 HTTP/2 API, replacing the legacy batch IID endpoint with individual requests per registration token. Feedback focuses on refactoring the asynchronous logic to avoid the new Promise constructor anti-pattern, ensuring the HTTP/2 session is established before requests are sent, and using defined constants instead of hardcoded host strings for better maintainability.

[gemini-code-assist]

The current implementation uses the new Promise constructor anti-pattern to wrap existing promise-returning logic. Additionally, it initiates all HTTP requests immediately, even if the HTTP/2 session invocation fails, which is inefficient and can lead to unhandled rejections.

Refactoring this into a clean promise chain ensures that requests are only started after the session is successfully established and simplifies error handling.

returnhttp2SessionHandler.invoke().catch((error)=>{thrownewFirebaseMessagingSessionError(error,undefined,undefined);}).then(()=>{constrequests=registrationTokensArray.map(async(registrationId)=>{letrequestPath=`/v1/projects/${projectId}/registrations/${registrationId}/topicSubscriptions`;if(isSubscribe){requestPath+=`?topic_name=${topicName}`;}else{requestPath+=`/${topicName}?allow_missing=true`;}returnthis.messagingRequestHandler.invokeHttp2RequestHandler(FCM_SEND_HOST,requestPath,httpMethod,isSubscribe ? {} : undefined,http2SessionHandler);});returnPromise.allSettled(requests);}).then((results)=>{if(results.length>0&&results.every((r)=>r.status==='rejected')){constfirstReason=(results[0]asPromiseRejectedResult).reason;if(firstReasoninstanceofRequestResponseError){throwcreateFirebaseError(firstReason);}else{throwfirstReason;}}constresponse: MessagingTopicManagementResponse={successCount: 0,failureCount: 0,errors: [],};results.forEach((result,index)=>{if(result.status==='fulfilled'){response.successCount+=1;}else{response.failureCount+=1;consterr=result.reason;consterrorCode=err.response?.isJson() ? getErrorCode(err.response.data) : null;consterrorMessage=err.response?.isJson() ? err.response.data?.error?.message : err.message;constnewError=FirebaseMessagingError.fromTopicManagementServerError(errorCode||'UNKNOWN',errorMessage,err.response?.isJson() ? err.response.data : undefined);response.errors.push({
index,error: newError,});}});returnresponse;}).finally(()=>{http2SessionHandler.close();});

[gemini-code-assist]

Avoid hardcoding the host string. Use the FCM_SEND_HOST constant defined at the top of the file for consistency.

Suggested change

	consthttp2SessionHandler=newHttp2SessionHandler('https://fcm.googleapis.com');
	consthttp2SessionHandler=newHttp2SessionHandler(`https://${FCM_SEND_HOST}`);