fix: detect .txt files as Markdown instead of rejecting them

[lawrence3699]

Summary

Fixes #3259

PR #3161 added "txt" to FormatToExtensions[InputFormat.MD], but the content-based disambiguation in _guess_from_content was never updated to handle the new case. When a .txt file enters _guess_format:

1. _mime_from_extension("txt") returns None (intentionally, because "txt" is in both the XML_USPTO and MD extension lists).
2. The content-detection chain produces mime = "text/plain".
3. _guess_from_content checks for USPTO format (PATN\r\n header), finds nothing, and returns None.
4. _guess_format returns None → converter raises "File format not allowed".

The fix adds an elif InputFormat.MD in formats fallback so non-USPTO text/plain content resolves to InputFormat.MD. USPTO .txt files are unaffected because the existing if check runs first.

Changes

• docling/datamodel/document.py: add MD fallback in _guess_from_content for text/plain
• tests/test_input_doc.py: update expectations for non-USPTO .txt files (None → InputFormat.MD) and add a plain .txt detection case

Checks

• pytest tests/test_input_doc.py — 8/8 passed

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[Copilot]

Pull request overview

This PR fixes .txt input handling in Docling’s format-guessing logic so that non‑USPTO plain-text files are detected as Markdown (MD) instead of being rejected, while preserving the existing USPTO .txt detection via the PATN\r\n header.

Changes:

• Add an MD fallback in _guess_from_content() for text/plain when USPTO content is not detected.
• Update and extend _guess_format tests to expect .txt → InputFormat.MD for non‑USPTO plain text.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
docling/datamodel/document.py	Adds text/plain disambiguation fallback to InputFormat.MD when content is not USPTO.
tests/test_input_doc.py	Updates assertions for non‑USPTO .txt and adds a new plain .txt detection test.

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
docling/datamodel/document.py	0.00%	2 Missing ⚠️

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[PeterStaar-IBM]

@lawrence3699 Can you please:

1. fix the tests
2. run a few times uv run pre-commit run --all-files (after you fix the tests?)