chore: officially migrate to submodule (#4502)

* remove apps/core and apps/fern

* fix precommit

* add submodule updates in workflows

* submodule

* remove core tests

* update core revision

* Add submodules: true to all GitHub workflows

- Ensure all workflows can access git submodules
- Add submodules support to deployment, test, and CI workflows
- Fix YAML syntax issues in workflow files

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* remove core-lint

* upgrade core with latest main of oss

---------

Co-authored-by: Claude <noreply@anthropic.com>

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

@@ -1,48 +0,0 @@
-name: 🐍🧪 [Core] Send Message SDK Tests
-
-on:
-  pull_request:
-    branches:
-      - main
-  pull_request_target:
-    branches:
-      - main
-    types: [labeled]
-
-concurrency:
-  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
-  cancel-in-progress: ${{ github.ref != 'refs/heads/main' }}
-
-jobs:
-  send-message-tests:
-    # Run on pull_request OR on pull_request_target only when labeled "safe to test"
-    if: github.event_name == 'pull_request' || (github.event_name == 'pull_request_target' && contains(github.event.pull_request.labels.*.name, 'safe to test'))
-    uses: ./.github/workflows/reusable-test-workflow.yml
-    with:
-      test-type: 'send-message'
-      changed-files-pattern: |
-        apps/core/**
-        .github/workflows/reusable-test-workflow.yml
-        .github/workflows/send-message-integration-tests.yml
-      install-args: '--extra dev --extra postgres --extra external-tools --extra cloud-tool-sandbox --extra google --extra redis'
-      timeout-minutes: 15
-      runner: '["self-hosted", "medium"]'
-      ref: ${{ github.event.pull_request.head.sha || github.sha }}
-      use-redis: true
-      # TODO: "azure-gpt-4o-mini.json" add back later, getting content violation
-      matrix-strategy: |
-        {
-          "fail-fast": false,
-          "matrix": {
-            "config_file": [
-              "openai-gpt-4o-mini.json",
-              "claude-4-sonnet-extended.json",
-              "claude-3-5-sonnet.json",
-              "claude-3-7-sonnet-extended.json",
-              "gemini-1.5-pro.json",
-              "gemini-2.5-pro.json",
-              "gemini-2.5-flash.json"
-            ]
-          }
-        }
-    secrets: inherit