extend allow-prereleases gating to all wheel-install + min-version steps

Initial PR only gated the two `Import dist package` steps. CI surfaced
that the same flag is needed in three more places that hit the same
transitive-prerelease resolution:

- `Run unit tests with minimum dependency versions` — both
  `uv pip install --force-reinstall --editable .` and the
  `--force-reinstall $MIN_VERSIONS` call.
- `Test against ${{ matrix.partner }}` — the
  `uv pip install ../../core/dist/*.whl` step that overrides the
  installed core with the locally-built wheel.
- `Install ${{ matrix.package.name }} with local packages`
  (test-dependents job, e.g. deepagents) — the
  `uv pip install $GITHUB_WORKSPACE/dist/*.whl` override step.

Each step gets its own
`PRERELEASE_FLAG: ${{ inputs.allow-prereleases && '--prerelease=allow' || '' }}`
env so the flag is empty when `allow-prereleases` is `false` (the
default) — no behavior change for stable releases.

.github/workflows/_release.yml

@@ -407,9 +407,10 @@ jobs:
         if: ${{ steps.min-version.outputs.min-versions != '' }}
         env:
           MIN_VERSIONS: ${{ steps.min-version.outputs.min-versions }}
+          PRERELEASE_FLAG: ${{ inputs.allow-prereleases && '--prerelease=allow' || '' }}
         run: |
-          VIRTUAL_ENV=.venv uv pip install --force-reinstall --editable .
-          VIRTUAL_ENV=.venv uv pip install --force-reinstall $MIN_VERSIONS
+          VIRTUAL_ENV=.venv uv pip install $PRERELEASE_FLAG --force-reinstall --editable .
+          VIRTUAL_ENV=.venv uv pip install $PRERELEASE_FLAG --force-reinstall $MIN_VERSIONS
           make tests PYTEST_EXTRA="-q -k 'not test_serdes'"
         working-directory: ${{ env.EFFECTIVE_WORKING_DIR }}
 
@@ -519,6 +520,8 @@ jobs:
 
       - name: Test against ${{ matrix.partner }}
         if: startsWith(env.EFFECTIVE_WORKING_DIR, 'libs/core')
+        env:
+          PRERELEASE_FLAG: ${{ inputs.allow-prereleases && '--prerelease=allow' || '' }}
         run: |
           # Identify latest tag, excluding pre-releases
           LATEST_PACKAGE_TAG="$(
@@ -548,7 +551,7 @@ jobs:
 
           # Run tests
           uv sync --group test --group test_integration
-          uv pip install ../../core/dist/*.whl
+          uv pip install $PRERELEASE_FLAG ../../core/dist/*.whl
           make integration_tests
 
   # Test external packages that depend on langchain-core/langchain against the new release
@@ -603,6 +606,8 @@ jobs:
       - name: Install ${{ matrix.package.name }} with local packages
         # External dependents don't have [tool.uv.sources] pointing to this repo,
         # so we install the package normally then override with the built wheel.
+        env:
+          PRERELEASE_FLAG: ${{ inputs.allow-prereleases && '--prerelease=allow' || '' }}
         run: |
           cd ${{ matrix.package.name }}/${{ matrix.package.path }}
 
@@ -610,7 +615,7 @@ jobs:
           uv sync --group test
 
           # Override with the built wheel from this release
-          uv pip install $GITHUB_WORKSPACE/dist/*.whl
+          uv pip install $PRERELEASE_FLAG $GITHUB_WORKSPACE/dist/*.whl
 
       - name: Run ${{ matrix.package.name }} tests
         run: |