ci: retire openai_cross_version_test + langchain_cross_version_test

Both jobs ran current CI against the frozen `integrations/openai` /
`integrations/langchain` source from historical tags (databricks-ai-
v0.2.0 through v0.5.0). They worked while PyPI dep evolution was
backward-compatible.

That contract broke with the databricks-vectorsearch → databricks-ai-
search rename. The frozen historical sources import VectorSearchIndex
from `databricks.vector_search.client` (the path that was removed in
the rename), and there's no PR to HEAD that can update that frozen
source. The matrix is permanently red regardless of what HEAD does.

Per Ann: "we can restart them from the version we are deploying."
Remove both jobs now; once we ship a release containing the ai-search
migration (#437), the cross-version matrix can be re-introduced with
that release as the new stable baseline.

The bridge's own unit tests + integration tests still cover regression
risk. This removes a non-Required CI signal that's now noise rather
than information.

Author: jennsun

.github/workflows/main.yml

@@ -106,52 +106,16 @@ jobs:
             --exact --group tests --extra memory \
             pytest --cov --cov-report=term tests/unit_tests
 
-  langchain_cross_version_test:
-    runs-on: ubuntu-latest
-    name: langchain_test (${{ matrix.python-version }}, ${{ matrix.version.name }})
-    strategy:
-      fail-fast: false
-      matrix:
-        python-version: ["3.10"]
-        version:
-          - { ref: "databricks-ai-v0.5.0", name: "v0.5.0" }
-          - { ref: "databricks-ai-v0.4.0", name: "v0.4.0" }
-          - { ref: "databricks-ai-v0.3.0", name: "v0.3.0" }
-          - { ref: "databricks-ai-v0.2.0", name: "v0.2.0" }
-    timeout-minutes: 20
-    steps:
-      - name: Checkout code
-        uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5
-      - name: Set up Python
-        uses: actions/setup-python@a26af69be951a213d495a4c3e4e4022e16d87065
-        with:
-          python-version: ${{ matrix.python-version }}
-      - name: Install dependencies
-        run: |
-          pip install .
-      - name: Checkout langchain version
-        uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5
-        with:
-          ref: ${{ matrix.version.ref }}
-          fetch-depth: 1
-          path: older-version
-      - name: Replace langchain with older version
-        run: |
-          # Remove current langchain if it exists to avoid conflicts
-          rm -rf integrations/langchain
-
-          # Copy older version of langchain to the main repo
-          cp -r older-version/integrations/langchain integrations/
-      - name: Install langchain dependency
-        run: |
-          pip install integrations/langchain --group dev
-      - name: Run tests
-        run: |
-          # Only testing initialization since functionality can change
-          pytest integrations/langchain/tests/unit_tests/test_vector_search_retriever_tool.py::test_init
-          pytest integrations/langchain/tests/unit_tests/test_genie.py
-          pytest integrations/langchain/tests/unit_tests/test_embeddings.py
-          pytest integrations/langchain/tests/unit_tests/test_chat_models.py
+  # `langchain_cross_version_test` and `openai_cross_version_test` (previously
+  # defined here) ran HEAD's CI against the frozen `integrations/langchain` /
+  # `integrations/openai` source from historical tags (databricks-ai-v0.2.0
+  # through v0.5.0) against current PyPI. After `databricks-vectorsearch>=0.74`
+  # was renamed to `databricks-ai-search` and removed the `VectorSearchIndex`
+  # re-export from `databricks.vector_search.client`, those frozen historical
+  # sources cannot import and the matrix is permanently red regardless of
+  # what HEAD does. Both jobs are removed; cross-version testing will be
+  # re-seeded once a new release containing the ai-search migration ships
+  # and we have a fresh stable baseline to test backward compat against.
 
   openai_test:
     runs-on: ubuntu-latest
@@ -198,48 +162,6 @@ jobs:
             --exact --group tests --extra memory \
             pytest --cov --cov-report=term tests/unit_tests
 
-  openai_cross_version_test:
-    runs-on: ubuntu-latest
-    name: openai_test (${{ matrix.python-version }}, ${{ matrix.version.name }})
-    strategy:
-      fail-fast: false
-      matrix:
-        python-version: ["3.10"]
-        version:
-          - { ref: "databricks-ai-v0.5.0", name: "v0.5.0", mlflow: "mlflow<3" }
-          - { ref: "databricks-ai-v0.4.0", name: "v0.4.0", mlflow: "mlflow<3" }
-          - { ref: "databricks-ai-v0.3.0", name: "v0.3.0", mlflow: "mlflow<3" }
-    timeout-minutes: 20
-    steps:
-      - name: Checkout code
-        uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5
-      - name: Set up Python
-        uses: actions/setup-python@a26af69be951a213d495a4c3e4e4022e16d87065
-        with:
-          python-version: ${{ matrix.python-version }}
-      - name: Checkout openai version
-        uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5
-        with:
-          ref: ${{ matrix.version.ref }}
-          fetch-depth: 1
-          path: older-version
-      - name: Replace openai with older version
-        run: |
-          # Remove current openai if it exists to avoid conflicts
-          rm -rf integrations/openai
-
-          # Copy older version of openai to the main repo
-          cp -r older-version/integrations/openai integrations/
-      - name: Install openai dependency
-        run: |
-          pip install .
-          pip install integrations/openai --group dev
-          pip install "${{ matrix.version.mlflow }}"
-      - name: Run tests
-        run: |
-          # Only testing initialization since functionality can change
-          pytest integrations/openai/tests/unit_tests/test_vector_search_retriever_tool.py::test_vector_search_retriever_tool_init
-
   llamaindex_test:
     runs-on: ubuntu-latest
     name: "llamaindex_test (python: ${{ matrix.python-version }}, uv: ${{ matrix.uv-resolution }})"