[ci feat] Support /bot run tests/<dir> to scope CI test runs (#3422)

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## 📌 Description

### Note
Relies on upstream internal gitlab changes being merged first.

### Summary
- Add --test-path argument to parse_args() in test_utils.sh and honor
TEST_PATH env var (set by GitLab CI pipeline variable)
- Scope find in task_run_unit_tests.sh to the specified subdirectory
instead of all of tests/
- Validate the path exists upfront and list available directories on
failure

### Motivation
The full test suite (attn + moe + gdn + ...) exceeds the 4-hour CI
timeout. This allows running a single test subdirectory via /bot run
tests/moe, keeping each scoped run well within the window while we work
on pruning the full suite.

### Usage
When TEST_PATH is set (via env var or --test-path flag), only tests
under that directory are discovered. Without it, behavior is unchanged.
```
/bot run tests/moe       # only MOE tests
/bot run tests/attention # only attention tests
/bot run                 # all tests (unchanged)
```

### Test plan

- [ ] Verify /bot run without a path still runs the full test suite
 - [ ]  Verify /bot run tests/moe scopes discovery to tests/moe/ only
- [ ] Verify an invalid path (e.g. tests/nonexistent) fails early with a
helpful error
- [ ] Verify --test-path flag works for local runs: bash
scripts/task_run_unit_tests.sh --test-path tests/nor

## 🔍 Related Issues

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request, please make sure the following items are complete.

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## 🧪 Tests

- [ ] Tests have been added or updated as needed.
- [ ] All tests are passing (`unittest`, etc.).

## Reviewer Notes

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## Summary by CodeRabbit

* **Chores**
* Test discovery is now configurable via the TEST_PATH environment
variable or --test-path CLI argument, enabling scoped test execution.
* Log messages now reflect the selected search directory; overall test
execution behavior remains unchanged.
* **Bug Fixes / UX**
* Added validation and clearer error messages when the configured test
directory doesn't exist, including listing available top-level test
directories.
* **Style**
  * Displays a “SCOPED TEST MODE” banner when a test path is set.

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