fix: omit description heading for custom image prompts

[VictorMaxWang]

Summary

• Respect non-empty custom image prompts by omitting MarkItDown's hardcoded English # Description: heading.
• Preserve the existing heading and default caption prompt when llm_prompt is None, empty, or whitespace-only.

Reproduction / root cause

ImageConverter correctly passed llm_prompt to the LLM client, but then unconditionally prepended # Description: to the returned caption. As a result, callers requesting localized or heading-free output still received an English heading.

The regression test uses the repository's existing test_llm.jpg and a MagicMock client, so it exercises the public MarkItDown.convert() path without credentials or network access.

Changes

• Reuse the resolved llm_prompt when generating the image description.
• Add the default heading only when the converter is also using its default prompt semantics.
• Cover None, empty, whitespace-only, and non-empty custom prompts with a parameterized regression test.

Tests

• hatch test tests/test_module_misc.py -k 'test_markitdown_llm_prompt_controls_default_heading or test_markitdown_llm_parameters' -vv: passed (5 passed, 14 deselected).
• Offline MarkItDown.convert() probe with test_llm.jpg and a mocked client: passed; custom output was \nBeschreibung auf Deutsch.\n, while default output remained \n# Description:\nDefault caption.\n.
• $testEnv = hatch env find hatch-test.py3.13; $env:PYTHONPATH = (Resolve-Path 'src').Path + ';' + (Join-Path $testEnv 'Lib\site-packages'); $env:GITHUB_ACTIONS = 'true'; $env:PYTHONUTF8 = '1'; hatch test: 309 passed, 34 skipped, 1 failed. The remaining failure is the existing Windows-only test_file_uris POSIX-path assertion (/path/to/file.txt vs D:\path\to\file.txt); this PR does not modify URI handling.
• pre-commit run --all-files: passed.

Security / compatibility considerations

• No file, URI, network-target, archive, or resource-limit validation is changed.
• No dependencies are added.
• Existing output is preserved for default, empty, and whitespace-only prompts; only non-empty custom prompts stop receiving the library-injected English heading.
• No real LLM request or external credential is used by the regression test.

Fixes #315

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