fix: MarkdownHeaderSplitter silently drops a trailing header with no body text

[lntutor]

What's broken

With keep_headers=True (the default), MarkdownHeaderSplitter buffers a header whose only content is whitespace into pending_headers and prepends it to the next content chunk. But a header (or run of headers) at the end of the document has no next chunk, so the buffer is never flushed and the header text is dropped:

text = "# Header 1\nContent 1.\n# Header 2\n"
docs = MarkdownHeaderSplitter().run(documents=[Document(content=text)])["documents"]
"".join(d.content for d in docs)   # '# Header 1\nContent 1.\n'  -- '# Header 2' is gone

This breaks the reconstruction invariant the existing tests assert (test_basic_split, test_keep_headers_preserves_parent_headers_for_first_child both check "".join(doc.content) == original), and it's inconsistent: a middle empty header is preserved (prepended to the next chunk), a trailing one is dropped.

Fix

Track the start offset / text / level of buffered trailing headers and, before returning, flush them as a final chunk sliced from the original text (so byte-level reconstruction and downstream page counting stay exact) — mirroring how a middle empty header is already preserved.

Distinct from #11920 (content dropped before an embedded header/code-fence).

Tests

Adds test_trailing_header_without_content_is_not_dropped. Full test/components/preprocessors/test_markdown_header_splitter.py passes (35). Release note included.

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