fix: accept a path-like tool_index_path, as weights already does

[kevin9327]

Problem

Needle(tool_index_path=Path("tools.idx")) raises, but only when no tuned weights are passed:

AttributeError: 'PosixPath' object has no attribute 'encode'

The same argument is accepted a few lines further down. __init__ has two paths and they disagree:

self._weights = os.fspath(weights) if weights is not None else None          # path-like ok
...
self._tool_index_path = tool_index_path.encode("utf-8") if tool_index_path else None   # str only
...
    self._worker = FineTuneWorker(
        ..., os.fspath(tool_index_path) if tool_index_path else None, ...)   # path-like ok

So with weights= the agent takes a Path and without it the same call fails. weights itself
takes one either way, which is what a caller reasonably expects of the argument beside it, and
Path is the ordinary way to build a path next to the checkpoint you just wrote.

Fix

One line: os.fspath before the encode, matching weights directly above and the worker branch
directly below.

Tests

Two in tests/test_weights.py, on the existing engine fixture so no engine is needed:

• test_tool_index_path_takes_a_path_object — a Path reaches the engine as its os.fspath bytes.
  Before this change it raises the AttributeError above.
• test_tool_index_path_agrees_between_the_base_and_tuned_paths — the same Path is accepted by
  the base agent and by the worker-backed tuned agent, which is the asymmetry that made this easy to
  miss.

How I tested

Windows 11, Python 3.12.10, pip install -e ".[test,train]".
pytest -q -m "not slow" gives 66 passed, 9 skipped; the two new tests are the only change and
nothing else moves. The 9 skips are the six engine tests and the three worker tests that want a C
compiler.

test_lora.py, test_render.py, test_run.py, test_build.py and test_finetune.py are excluded
from that run: they need flax, whose orbax-checkpoint dependency will not install on Windows
because its paths exceed MAX_PATH. Nothing here touches them, and CI runs them on Ubuntu.