Use “built-in” in generated documentation for builtin_tools.py

[coderabbitai]

Summary

Change the source description for builtin_tools.py from “builtin” to “built-in”, then regenerate the affected README documentation.

Rationale

src/utils/README.md is generated by scripts/gen_doc.py. Editing only the generated README can cause a later generation to restore the current wording.

Affected areas

• scripts/gen_doc.py input source or metadata that provides the description for builtin_tools.py
• src/utils/README.md

Required changes

1. Update the source description to use “built-in” in “Discover built-in file-search tools when that provider is configured.”
2. Regenerate src/utils/README.md.
3. Verify that the generated README preserves the corrected wording.

Acceptance criteria

• The source description uses “built-in”.
• src/utils/README.md contains the regenerated description with “built-in”.
• A subsequent documentation generation does not restore “builtin”.

Backlinks

• Pull request: LCORE-3444: Properly formatted devel doc #2367
• Review comment: LCORE-3444: Properly formatted devel doc #2367 (comment)

Requested by: @tisnik