BUG: more core fixes (#13665)

Fix some circular deps:
- move PromptValue into top level module bc both PromptTemplates and
OutputParsers import
- move tracer context vars to `tracers.context` and import them in
functions in `callbacks.manager`
- add core import tests

.github/workflows/langchain_ci.yml

@@ -63,6 +63,46 @@ jobs:
       langchain-core-location: ../core
     secrets: inherit
 
+  # It's possible that langchain works fine with the latest *published* langchain-core,
+  # but is broken with the langchain-core on `master`.
+  #
+  # We want to catch situations like that *before* releasing a new langchain-core, hence this test.
+  test-with-latest-langchain-core:
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: ${{ env.WORKDIR }}
+    strategy:
+      matrix:
+        python-version:
+          - "3.8"
+          - "3.9"
+          - "3.10"
+          - "3.11"
+    name: test with unpublished langchain-core - Python ${{ matrix.python-version }}
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Python ${{ matrix.python-version }} + Poetry ${{ env.POETRY_VERSION }}
+        uses: "./.github/actions/poetry_setup"
+        with:
+          python-version: ${{ matrix.python-version }}
+          poetry-version: ${{ env.POETRY_VERSION }}
+          working-directory: ${{ env.WORKDIR }}
+          cache-key: unpublished-langchain-core
+
+      - name: Install dependencies
+        shell: bash
+        run: |
+          echo "Running tests with unpublished langchain, installing dependencies with poetry..."
+          poetry install
+
+          echo "Editably installing langchain-core outside of poetry, to avoid messing up lockfile..."
+          poetry run pip install -e ../core
+
+      - name: Run tests
+        run: make test
+
   extended-tests:
     runs-on: ubuntu-latest
     defaults: