Specify cli root_dir as Path to avoid str / str operation

[olivialynn]

Passing --lbench-root crashes at startup with TypeError: unsupported operand type(s) for /: 'str' and 'str'

Currently:
We have 2 ways to set the root dir:

• via CLI (doesn't work; interprets it as a string)
• get_lbench_root_dir (works; parses it as a Path from LBENCH_ROOT env var)

In src/lbench/pytest/plugin.py, we call:

group.addoption("--lbench-root", action="store", default=None, ...)   # no type → value is a str
...
root = config.getoption("--lbench-root")      # e.g. "/some/dir"  (str)
if root is None:
    root = get_lbench_root_dir()              # Path; only this branch works
run_dir = root / run_id                       # str / str → TypeError

Proposed change:
Adding type=Path to the "--lbench-root" option
This lets us specify the root_dir via the cli, and still construct the run_dir path construction via the / operator.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 3.47%. Comparing base (a214c4a) to head (b9fc38b).

Files with missing lines	Patch %	Lines
src/lbench/pytest/plugin.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##            main     #21      +/-   ##
========================================
- Coverage   3.48%   3.47%   -0.01%     
========================================
  Files         26      26              
  Lines       1062    1064       +2     
========================================
  Hits          37      37              
- Misses      1025    1027       +2     

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[smcguire-cmu]

LGTM