fix(ci): allow pre-releases when pinning the nightly in migration validation (#13599)

Migration Test: pip/venv (stable -> nightly) failed deterministically on
nightly run 27260425158 (twice, including a rerun):

  hint: langflow-base was requested with a pre-release marker (e.g.,
  langflow-base==1.11.0.dev1), but pre-releases weren't enabled
  (try: --prerelease=allow)

This is the first nightly publishing as a canonical .devN pre-release
of the langflow distribution (nightly -> stable bundle cutover). The
'latest' branch of the upgrade step already passes --prerelease=allow,
but the pinned-version branches do not. uv implicitly allows the
pre-release for the directly requested ==X.Y.Z.devN pin, yet langflow's
metadata pins langflow-base==X.Y.Z.devN transitively, and uv rejects
transitive pre-releases unless they are enabled - so the install fails
after the stable uninstall, sinking the migration test.

Add --prerelease=allow to both pinned-version install lines.

Author: erichare

.github/workflows/db-migration-validation.yml

@@ -182,13 +182,16 @@ jobs:
               # Install latest nightly (canonical pre-release) from PyPI
               uv pip install --upgrade --prerelease=allow 'langflow[postgresql]'
             else
-              # Install specific version
-              uv pip install --upgrade "langflow[postgresql]==$VERSION"
+              # Install specific version. --prerelease=allow is required even for an
+              # exact ==X.Y.Z.devN pin: uv only implicitly allows the pre-release for
+              # the directly requested package, while langflow's metadata pins
+              # langflow-base==X.Y.Z.devN transitively, which uv rejects without it.
+              uv pip install --upgrade --prerelease=allow "langflow[postgresql]==$VERSION"
             fi
           else
             # Direct version string (strip 'v' prefix if present)
             VERSION="${NIGHTLY_TAG#v}"
-            uv pip install --upgrade "langflow[postgresql]==$VERSION"
+            uv pip install --upgrade --prerelease=allow "langflow[postgresql]==$VERSION"
           fi
 
           # Verify upgrade