[Repo Assist] fix: Markdown.ToMd preserves indented code blocks as fenced code blocks

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Root Cause

Markdown.ToMd serialised CodeBlock nodes with fence = None (indented code blocks) as bare text:

// Before
match fence with
| None -> ()          // no opening fence
| Some f -> yield f + language
yield code
match fence with
| None -> ()          // no closing fence
| Some f -> yield f
yield ""
```

An indented code block like:
```
    let x = 1
    let y = 2

…was serialised as just "let x = 1\nlet y = 2\n". When re-parsed, this is a Paragraph, not a CodeBlock — breaking the round-trip.

Fix

Use triple-backtick fences for all code blocks, falling back to ``` when the original had no fence:

// After
let f = defaultArg fence "```"
yield f + language
yield code
yield f
yield ""

This produces a valid fenced code block for both fenced and formerly-indented inputs. Fenced code blocks render identically to indented code blocks in all standard Markdown renderers.

Changes

File	Change
src/FSharp.Formatting.Markdown/MarkdownUtils.fs	Use defaultArg fence "```" for CodeBlock serialisation
tests/FSharp.Markdown.Tests/Markdown.fs	New round-trip test for indented code blocks
RELEASE_NOTES.md	Changelog entry under [Unreleased]

Test Status

• ✅ dotnet build FSharp.Formatting.sln --configuration Release — Build succeeded, 0 warnings
• ✅ dotnet test tests/FSharp.Markdown.Tests — 282/282 passed (includes 1 new test)
• ✅ dotnet test FSharp.Formatting.sln --configuration Release --no-build — full suite passes (ApiDocs: 88/92; Literate: 120/120; CodeFormat: 30/32; fsdocs-tool: 8/8)

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