[Data] Fix: Replace bare raise with TypeError in string concatenation

[slfan1989]

Description

This PR fixes a bug in _to_pa_string_input() where attempting to concatenate string columns with non-string columns (e.g., numeric types) would raise a bare RuntimeError instead of a descriptive TypeError.

Changes:

• Replaced bare raise statement with proper TypeError that includes a clear error message indicating expected vs actual input types
• Simplified control flow using early returns
• Added unit test test_string_concat_invalid_input_type to verify the fix

Before: Bare raise caused cryptic RuntimeError: No active exception to reraise

After: Clear TypeError: Expected string or string-like pyarrow Array/ChunkedArray for string concatenation, got int64.

Related issues

Link related issues: "Fixes #1234", "Closes #1234", or "Related to #1234".

Additional information

Example of the fixed behavior:

import pyarrow as pa
from ray.data.expressions import col

table = pa.table({"name": ["Alice", "Bob"], "age": [25, 30]})
expr = col("name") + col("age")  # Attempting to concat string with int

# Now raises: TypeError: Expected string or string-like pyarrow Array/ChunkedArray 
# for string concatenation, got int64.

[gemini-code-assist]

Code Review

This pull request is a solid improvement, replacing a bare raise with a descriptive TypeError for invalid string concatenation operations. The code is also simplified by using early returns. I've added one suggestion to make the error message even more informative by including the specific data type of the invalid input, which aligns with the goal stated in the pull request description. The new unit test is a great addition to prevent regressions.

[gemini-code-assist]

The error message can be made more specific to better align with the goal stated in the PR description. When x is a pyarrow.Array or pyarrow.ChunkedArray, using x.type instead of type(x).__name__ will provide the underlying data type (e.g., int64), which is more informative for debugging.

Suggested change

	raise TypeError(
	"Expected string or string-like pyarrow Array/ChunkedArray for string "
	f"concatenation, got {type(x).__name__}."
	)
	type_name = x.type if isinstance(x, (pa.Array, pa.ChunkedArray)) else type(x).__name__
	raise TypeError(
	"Expected string or string-like pyarrow Array/ChunkedArray for string "
	f"concatenation, got {type_name}."
	)

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

[goutamvenkat-anyscale]

Suggested cleanup:

    if isinstance(x, (pa.Array, pa.ChunkedArray)) and _is_pa_string_like(x):
        return _pa_decode_dict_string_array(x)
    actual_type = str(x.type) if isinstance(x, (pa.Array, pa.ChunkedArray)) else type(x).__name__
    raise TypeError(
        "Expected string or string-like pyarrow Array/ChunkedArray for string "
        f"concatenation, got {actual_type}."
    )

[slfan1989]

Thanks for the suggestions! I’ve updated the PR accordingly—could you please take another look?