[repo-assist] Fix front-matter parsing truncating values containing colons

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Summary (Task 5 — Coding Improvement)

FrontMatterFile.ParseFromLines in src/FSharp.Formatting.Common/Templating.fs parses YAML-like front matter (category, categoryindex, index) by splitting each line on : and keeping only the first two segments. Any additional : characters in a value were silently discarded — e.g. a title: field containing a colon (title: F#: An Introduction) would be captured as just "F#".

While title isn't currently consumed by this parser, the same bug would silently corrupt any future or custom front-matter field containing a colon, and this exact fix was previously implemented (and reviewed positively) in a since-closed PR (#1105) whose other, unrelated changes (nested navigation categories) ran into merge conflicts. This PR extracts just the safe, isolated colon-parsing fix plus regression tests, so it doesn't get lost.

Change

• FrontMatterFile.ParseFromLines: instead of first :: second :: _ -> Some(first, second), now joins all parts after the first : with String.Join(":", rest), preserving the full value.
• Added two unit tests in tests/FSharp.Literate.Tests/DocContentTests.fs:
  • Regression test confirming colons in values are preserved.
  • Confirms None is still returned when required fields (category/categoryindex/index) are missing.
• Added a RELEASE_NOTES.md entry under [Unreleased] / Fixed.

Test Status

• ✅ dotnet build FSharp.Formatting.sln --configuration Release — Build succeeded, 0 warnings, 0 errors.
• ✅ dotnet test tests/FSharp.Literate.Tests/FSharp.Literate.Tests.fsproj --configuration Release --no-build — all 145 tests passed (2 new).
• ✅ Formatted with dotnet fantomas.

Trade-offs

• Minimal, surgical change limited to the parsing helper and its tests; no behavior change for existing well-formed front matter without colons in values.

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