ci: SHA-pin all third-party GitHub Actions (#7277)

## Summary

- SHA-pins 7 distinct third-party actions across 10 workflow files to
full commit SHAs
- Prevents supply chain attacks via tag hijacking (mutable tags can be
force-pushed by a compromised maintainer account)
- Tag retained as an inline comment for readability

| Action | Before | After |
|--------|--------|-------|
| `dorny/paths-filter` | `@v4` | `@fbd0ab8...` |
| `Ana06/get-changed-files` | `@v2.3.0` | `@25f79e6...` |
| `docker/login-action` | `@v4` | `@b45d80f...` |
| `pypa/gh-action-pypi-publish` | `@release/v1` | `@ed0c539...` |
| `ncipollo/release-action` | `@v1` | `@339a818...` |
| `amannn/action-semantic-pull-request` | `@v6` | `@48f2562...` |
| `peter-evans/create-pull-request` | `@v8` | `@c0f553f...` |

## Test plan

- [x] CI passes on this PR
- [x] Verify each pinned action still functions (no behaviour change,
only ref format)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

.github/workflows/_test_langgraph.yml

@@ -31,7 +31,7 @@ jobs:
           cache-suffix: "test-langgraph"
           working-directory: libs/langgraph
       - name: Login to Docker Hub
-        uses: docker/login-action@v4
+        uses: docker/login-action@b45d80f862d83dbcd57f89517bcf500b2ab88fb2 # v4
         if: ${{ !github.event.pull_request.head.repo.fork }}
         with:
           username: ${{ secrets.DOCKERHUB_USERNAME }}