Fix StreamableContent render/sanitization order

[cstns]

Description

Fixed StreamableContent order in which markdown text is parsed, streamed and sanitized in order to correctly render markdown

Related Issue(s)

closes #6935

Checklist

• I have read the contribution guidelines
• Suitable unit/system level tests have been added and they pass
• Documentation has been updated
  • Upgrade instructions
  • Configuration details
  • Concepts
• Changes flowforge.yml?
  • Issue/PR raised on FlowFuse/helm to update ConfigMap Template
  • Issue/PR raised on FlowFuse/CloudProject to update values for Staging/Production
• Link to Changelog Entry PR, or note why one is not needed.

Labels

• Includes a DB migration? -> add the area:migration label

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.63%. Comparing base (5ff8364) to head (498ae0c).
⚠️ Report is 13 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6936   +/-   ##
=======================================
  Coverage   76.63%   76.63%           
=======================================
  Files         399      399           
  Lines       20167    20167           
  Branches     4856     4856           
=======================================
  Hits        15454    15454           
  Misses       4713     4713           

Flag	Coverage Δ
backend	76.63% <ø> (ø)

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