Bound markdown chunks

[abelfx]

Description

Fixes #369.

_chunk_markdown returned a heading-less file as one chunk and skipped MAX_CHUNK_CHARS. A file with headings could still exceed the cap when a single paragraph was longer than the limit, because the splitter only cuts on blank lines.

• A file with no # heading is now one section and goes through the same size pass as headed files.
• Anything still over MAX_CHUNK_CHARS after the paragraph split is cut on a newline or space, or mid-token when there is no whitespace.
• A warning is logged when a file has no headings, and when a character-level cut is applied.

How Has This Been Tested?

python3 -m pytest tests/test_rag_chunking.py

• A heading-less file larger than MAX_CHUNK_CHARS is split, and every chunk stays within the cap.
• The same text with one heading stays within the cap.
• One paragraph with no blank lines is cut.
• A string with no whitespace is cut mid-token and the pieces join back to the original text.
• Cuts land on word boundaries when spaces are present.
• Heading-less chunks keep the filename as the breadcrumb.
• A short heading-less file stays one chunk.
• Heading breadcrumbs (Top, Top > Nested, Second) are unchanged.
• Warnings are logged for a heading-less file and for a character-level cut.

Checklist

• PR contains autogenerated code
• Self-review completed
• Test scenarios above are passed with the version of the code from PR