[Repo Assist] test: expand Markdown.ToMd round-trip coverage with 15 new tests

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Summary

Adds 15 new tests for Markdown.ToMd serialisation, covering cases that had zero coverage prior to this PR.

New test cases

Area	Tests added
Heading levels	#### H4, ##### H5, ###### H6
Emphasis inside headings	## Hello *world*, ## Hello **world**
LaTeX display math	$$E = mc^2$$
Nested blockquote	"> > inner quote" round-trips with double > markers
Emphasis / code inside blockquote	"> *italic*", "> \printf`"`
Code block without language tag	bare ``` fence
Horizontal rule (dash variant)	---
Link with title attribute	[text](url "title")
Inline code with special chars	`a + b = c`
Nested unordered list	* outer\n\n * inner

All tests are self-contained — they call Markdown.Parse followed by Markdown.ToMd and assert on the serialised output.

Root Cause / Rationale

The ToMd serialiser was introduced as a first-class public API (used by the literate-programming pipeline and the fsdocs convert --output-format markdown path), but the test suite had minimal coverage of advanced features. These gaps were identified by code-review of MarkdownUtils.fs.

Trade-offs

Purely additive — no production code changed.

Test Status

✅ All 281 markdown tests pass (0 failures, 0 skipped).

Passed!  - Failed: 0, Passed: 281, Skipped: 0, Total: 281

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