fix(ci): stop uv run from half-reverting the bundle install it runs against (#14631)

The bundle-guarded lfx step installs lfx-bundles outside the lock on purpose, then
runs the tests with `uv run` -- which re-syncs the environment to uv.lock first.
That reverts the lock-managed packages the bundle install upgraded and leaves the
injected ones untouched, so the tests execute against a combination neither
resolution ever produced.

It stayed invisible until langchain-openai 1.5.2 shipped on 2026-08-18. The bundle
install resolved openai 1.5.2 with langchain-core 1.5.6; `uv run` put core back to
the locked 1.5.1; and 1.5.2 against 1.5.1 cannot import -- it reads
GATEWAY_METADATA_RESPONSE_KEY, which core added in 1.5.6. Both resolutions were
fine on their own. Pinning away from 1.5.2 would have hidden this until the next
release that needs a newer core, so the mix is what gets fixed.

Verified against the failing job locally: with `uv run`, core lands on 1.5.1 and
test_lmstudio_get_model_blocks_metadata fails exactly as CI reported; with
`uv run --no-sync`, core stays on 1.5.6 and all 8 tests pass.

cross-bundle-test.yml does the same install but invokes .venv/bin/python directly,
so nothing re-syncs there and it needs no change.