Improve exception for validate_key

[hendrikmakait]

• Closes Exception from validate_key is misleading for composite keys #10764
• Tests added / passed
• Passes pre-commit run --all-files

The exception now indicates the index of the invalid key type and the traceback reveals the actual problem. The latter is important for nested tuples.

Traceback (most recent call last):
  File "/Users/hendrikmakait/projects/dask/dask/dask/core.py", line 235, in validate_key
    validate_key(part)
  File "/Users/hendrikmakait/projects/dask/dask/dask/core.py", line 240, in validate_key
    raise TypeError(f"Unexpected key type {typ} ({key=!r})")
TypeError: Unexpected key type <class 'numpy.int64'> (key=2)

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/hendrikmakait/projects/dask/dask/dask/core.py", line 237, in validate_key
    raise TypeError(
TypeError: Composite key contains unexpected key type at _index=1 (key=(1, 2, 3))

[phofl]

lgtm

[phofl]

Suggested change

	f"Composite key contains unexpected key type at {_index=} ({key=!r})"
	f"Composite key contains unexpected key type at index={_index} ({key=!r})"

That would get ci passing

[hendrikmakait]

I shouldn't have followed flake8s advice there. Reverted to index and ignoring the check.

[phofl]

Sounds good to me