rest-api wrapper test: forward pyrealsense2 path to subprocess via PYTHONPATH

[Nir-Az]

Summary

• unit-tests/wrappers/rest-api/pytest-rest-api-wrapper.py runs the rest-api tests in a fresh python -m pytest subprocess. The parent pytest can import pyrealsense2 because unit-tests/conftest.py injects the locally-built .so directory into sys.path at runtime — but that's an in-process tweak the child interpreter doesn't see.
• Previously this worked in CI because the Jenkins pipeline did a system-wide pip install pyrealsense2. After that step was removed, the subprocess fails at collection with ModuleNotFoundError: No module named 'pyrealsense2' (e.g. LRS_linux_compile_lib_ci #11119).
• Fix: forward repo.find_pyrs_dir() to the subprocess via PYTHONPATH, prepended so it wins over anything else.

Test plan

• libci green on the rest-api wrapper test

[Copilot]

Pull request overview

This PR fixes rest-api wrapper subprocess test collection failures by ensuring the child python -m pytest process can import the locally-built pyrealsense2 (which is normally added to sys.path only in the parent pytest process via unit-tests/conftest.py).

Changes:

• Compute pyrs_dir = repo.find_pyrs_dir() in the wrapper test and forward it to the subprocess via PYTHONPATH.
• Pass the modified environment (env=...) into subprocess.run(...) so the child interpreter sees the path.

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[Copilot]

When PYTHONPATH is unset or empty, pyrs_dir + os.pathsep + "" leaves a trailing path separator, which introduces an empty entry in PYTHONPATH (interpreted as the current working directory on sys.path). This can change import precedence and reduce test isolation. Build PYTHONPATH so you only add os.pathsep + existing when an existing value is non-empty (otherwise set it to pyrs_dir alone).

Suggested change

	env["PYTHONPATH"] = pyrs_dir + os.pathsep + env.get("PYTHONPATH", "")
	existing_pythonpath = env.get("PYTHONPATH", "")
	env["PYTHONPATH"] = (
	pyrs_dir + os.pathsep + existing_pythonpath
	if existing_pythonpath
	else pyrs_dir
	)