fix: include file path in file errors

[nicholaspun-wandb]

Description

Returns the file path in the error message when a file operation fails

Testing

Updated unit tests and ran the following script:

from wandb.sandbox import Sandbox
import wandb

wandb.login()

with Sandbox.run(
    "sh",
    "-c",
    "sleep 60",
    container_image="python:3.11-slim",
    max_lifetime_seconds=120,
) as sb:
    print("sandbox_id", sb.sandbox_id)
    sb.wait(timeout=120)
    sb.read_file("/tmp/sandbox-e2e-missing-file.txt").result(timeout=60)

Before (only File operation failed displayed to the user):

uv run python missing_file_repro.py
wandb: [wandb.login()] Loaded credentials for https://api.wandb.ai from /Users/nicholaspun/.netrc.
wandb: Currently logged in as: nicholas-pun (wandb) to https://api.wandb.ai. Use `wandb login --relogin` to force relogin
sandbox_id b4be58e0-6257-4ce4-80be-689256bff5c8
Traceback (most recent call last):
  File "/Users/nicholaspun/Documents/test-wb-sandbox/.venv/lib/python3.11/site-packages/cwsandbox/_sandbox.py", line 3773, in _read_file_async
    response = await self._stub.RetrieveFile(
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   ...
    raise _translate_rpc_error(
cwsandbox.exceptions.SandboxFileError: File operation failed (CWSANDBOX_FILE_NOT_FOUND): peer Gateway returned RetrieveFile error: peer RetrieveFile returned error: rpc error: code = NotFound desc = 

After (File operation failed for '/tmp/sandbox-e2e-missing-file.txt' surfaced):

PYTHONPATH=/Users/nicholaspun/Documents/cwsandbox-client/src uv run python missing_file_repro.py
wandb: [wandb.login()] Loaded credentials for https://api.wandb.ai from /Users/nicholaspun/.netrc.
wandb: Currently logged in as: nicholas-pun (wandb) to https://api.wandb.ai. Use `wandb login --relogin` to force relogin
sandbox_id 3ca54f50-c94b-4e19-a8b4-d4fb9459bc3f
Traceback (most recent call last):
  File "/Users/nicholaspun/Documents/cwsandbox-client/src/cwsandbox/_sandbox.py", line 3905, in _read_file_unary_async
    response = await self._stub.RetrieveFile(
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  ...
  File "/Users/nicholaspun/Documents/cwsandbox-client/src/cwsandbox/_sandbox.py", line 3909, in _read_file_unary_async
    raise _translate_rpc_error(
cwsandbox.exceptions.SandboxFileError: File operation failed for '/tmp/sandbox-e2e-missing-file.txt' (CWSANDBOX_FILE_NOT_FOUND): peer Gateway returned RetrieveFile error: peer RetrieveFile returned error: rpc error: code = NotFound desc =