fix(core): add messages to bare raise ValueError calls

[Mason Daugherty (mdrxy)]

Closes #35727

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Several internal code paths raised a bare ValueError with no message, so when one of these conditions tripped, users saw a traceback with no explanation of what actually went wrong.

This adds descriptive messages to each of those raise ValueError calls:

• FewShotPromptWithTemplates._get_examples / _aget_examples — when neither examples nor example_selector is set.
• Prompt loading — when a referenced template file uses an unsupported (non-.txt) format.
• The LangSmith document loader — when both client and client_kwargs are supplied.
• The Anthropic file-search middleware brace expansion — for unbalanced or empty brace patterns.

Made by Open SWE

[codspeed-hq]

Merging this PR will not alter performance

✅ 13 untouched benchmarks
🆕 2 new benchmarks
⏩ 2 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
🆕	WallTime	test_convert_tools_cold	N/A	202.7 ms	N/A
🆕	WallTime	test_convert_tools_warm	N/A	2.8 ms	N/A

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_{Comparing mdrxy/core/bare-valueerror-messages (e0abd63) with master (36be77b)²}

Footnotes

1. 2 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

2. No successful run was found on master (3b48f48) during the generation of this report, so 36be77b was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩