fix(muya): preserve code fence length on serialize - #4755

Summary

A fenced code block written with more than three backticks — which is required when the block's own content contains a line — was always re-serialized (`getMarkdown` / source-mode toggle) with exactly three backticks. The inner then closed the block early and the saved markdown was corrupt.

Root cause: ICodeBlockState.meta had no fence-length field, markdownToState never captured it, and stateToMarkdown._serializeCodeBlock hard-coded ```.

Fix:

• capture the opening fence length on parse from the token's raw text, stored in meta.fenceLength only when it exceeds the default 3 (ordinary blocks keep their existing state shape)
• serialize a fence of max(3, stored, longestAllBacktickLineInBody + 1), so the output is both faithful to the original length and always long enough to be valid

Verification

Unit test src/state/__tests__/codeFenceLength.spec.ts, RED→GREEN:

• a 4-backtick block wrapping a ``` line keeps a ≥4 fence and round-trips without corruption
• byte-stable round trip
• ordinary 3-backtick blocks are unchanged

Full state suite + CommonMark/GFM conformance lock unaffected (2 unrelated parityExportHtml failures are a pre-existing worktree ?inline-CSS fs-deny, fail on clean develop too, pass in CI).

Fixes #1841