fix(cli): persist deployment name to .env after prompt (#7323)

## Summary
- When `langgraph deploy` prompts for the deployment name interactively,
persist it as `LANGSMITH_DEPLOYMENT_NAME` in the `.env` file so
subsequent deploys pick it up automatically
- Extracted `_resolve_env_path()` helper from `_parse_env_from_config()`
to share env file path resolution logic
- Skips writing when env vars are configured as an inline dict in
`langgraph.json` (no file to write to)
- **CI fix**: CLI integration tests were using `pip install` but
`setup-uv` with caching enabled, so the post-step cache save failed on a
non-existent directory — switched to `uv pip install`

## Test plan
- [x] All 207 existing unit tests pass
- [ ] Manual: run `langgraph deploy` without `--name`, enter a name at
prompt, verify `.env` now contains `LANGSMITH_DEPLOYMENT_NAME=<name>`
- [ ] Manual: run `langgraph deploy` again, verify the name is picked up
from `.env` without re-prompting
- [x] CI integration test cache fix (broken on main since Feb 2026)

.github/workflows/_integration_test.yml

@@ -49,7 +49,7 @@ jobs:
         uses: ./.github/actions/uv_setup
         with:
           python-version: ${{ matrix.python-version }}
-          cache-suffix: "cli-integration-test"
+          enable-cache: "false"
           working-directory: libs/cli
       - name: Install cli globally
         if: (steps.changed-files.outputs.all || github.event_name == 'workflow_dispatch')