chore: Move core tests into apps/core (OSS Migration) [LET-4169] (#4376)

* add a bunch of test to oss

* symlink and auto-detect dir

* symlink the other direction

* add pull_request_target logic

* remove undertaker and add alembic validation

* symlink doesn't work with gh actions and add validation workflow to ensure actions in cloud and oss are lockstep

* sync these

* specify extras selectively

.github/workflows/send-message-integration-tests.yaml

@@ -1,7 +1,7 @@
 name: Send Message SDK Tests
 on:
   pull_request_target:
-    # branches: [main] # TODO: uncomment before merge
+    branches: [main] # TODO: uncomment before merge
     types: [labeled]
     paths:
       - 'letta/**'