Fix: overlapping document parse race that can clear chunks

[6ba3i]

What problem does this PR solve?

This PR fixes a race in batch document parsing where overlapping parse requests for the same document could clear/rewrite chunk state and make previously parsed content appear lost. It adds an atomic per-document parse guard so only one parse can run at a time for that document (Fixes #13864 ).

Type of change

• Bug Fix (non-breaking change which fixes an issue)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.52%. Comparing base (dd52913) to head (132404f).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #13900   +/-   ##
=======================================
  Coverage   96.52%   96.52%           
=======================================
  Files          10       10           
  Lines         690      690           
  Branches      108      108           
=======================================
  Hits          666      666           
  Misses          8        8           
  Partials       16       16           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.