fix(ci): use subprocess for Triton compile helpers (#28775)
## Summary
This PR replaces shell-based command execution in the Triton compile
helper with `subprocess.run()`, and adds regression tests to lock in
that behavior.
## Key Changes
- Switched `tools/ci_build/compile_triton.py` to use `subprocess.run()`
for `objcopy` and `ar` execution instead of `os.system()`.
- Added `tools/ci_build/test_compile_triton.py` to verify the helper
functions invoke subprocess-based command execution and preserve the
expected working directory and error handling.
- Hardened directory handling in the Triton compile path so the output
directory is created safely with `os.makedirs(..., exist_ok=True)`.
## Why
The previous shell-based implementation was fragile and harder to test.
Using `subprocess.run()` makes the command execution path explicit,
easier to validate, and safer for CI and cross-platform invocation.
## Testing
- `source .venv/bin/activate && python -m unittest
tools.ci_build.test_compile_triton`
- `source .venv/bin/activate && lintrunner -a`
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