[Repo Assist] Make MarkdownRange non-optional in MarkdownSpan and MarkdownParagraph (fixes #742)

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🤖 This PR was created by Repo Assist, an automated AI assistant, in response to a /repo-assist command on issue #742.

Summary

Closes #742

This PR makes the range field non-optional in all MarkdownSpan and MarkdownParagraph discriminated union cases, as requested by @dsyme and originally reported by @artempyanykh.

Previously, every Markdown AST node had range: MarkdownRange option. Now all cases have range: MarkdownRange, with MarkdownRange.zero used as a fallback where no source range is available.

Root Cause

The range field was originally defined as optional throughout the AST. The one place where it became None during parsing was in updateCurrentRange when the line list was empty. This has now been fixed to return MarkdownRange.zero instead.

Changes

• MarkdownModel.fs – All range: MarkdownRange option → range: MarkdownRange in MarkdownSpan and MarkdownParagraph DU cases; None → MarkdownRange.zero in Dsl module
• MarkdownInlineParser.fs – CurrentRange field simplified from option to plain MarkdownRange; all option-matching removed from range computations
• MarkdownBlockParser.fs – updateCurrentRange returns MarkdownRange directly (with MarkdownRange.zero for empty input)
• Markdown.fs – Removed Some() wrapper from initial CurrentRange
• Transformations.fs, ParseScript.fs, Formatting.fs, Literate.fs – All range None args replaced with MarkdownRange.zero
• tests/ – Updated test expectations: Some({StartLine = ...; ...}) → {StartLine = ...; ...}
• RELEASE_NOTES.md – Added changelog entry

Breaking Change

This is a breaking API change for any code that:

• Pattern-matches on MarkdownSpan or MarkdownParagraph with named range fields
• Constructs MarkdownSpan or MarkdownParagraph values with explicit range arguments
• Uses range.Value or matches Some r/None on the range field

Migration: replace range: MarkdownRange option → range: MarkdownRange, and any None range → MarkdownRange.zero.

Test Status

Build: ✅ Passed
Tests: ✅ All 257 tests passed

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