refactor: resolve function regions during build phase to fix VPC connectors (#10471)

* Attempt to resolve function regions earlier before used for VPC.

* lint fixes

* Check for REGION_TBD, update tests

* Add CHANGELOG entry

---------

Co-authored-by: Andy Perelson <ajp@google.com>

Author: wandamora

CHANGELOG.md

@@ -1 +1,2 @@
 - Updated Pub/Sub emulator to version 0.8.31
+- Resolves undefined regions earlier, during the build to backend resolution phase (#10471)

src/deploy/functions/build.ts

@@ -257,8 +257,8 @@ export type Endpoint = Triggered & {
   // Defaults to the compute service account when a function is first created as a GCF gen 2 function.
   serviceAccount?: ServiceAccount | Expression<string> | null;
 
-  // defaults to ["us-central1"], overridable in firebase-tools with
-  //  process.env.FIREBASE_FUNCTIONS_DEFAULT_REGION
+  // Defaults to REGION_TBD. The deployment region is resolved dynamically at deploy-time
+  // based on event trigger sources or matching existing functions, falling back to "us-central1".
   region?: ListField;
 
   // The Cloud project associated with this endpoint.