fix(agents-server): sanitize attachment filename fallback

[KyleAMathews]

Summary

• sanitize the legacy Content-Disposition filename fallback to ASCII-only before setting attachment response headers
• preserve the full Unicode filename in the RFC 5987 filename* parameter
• add a regression test for screenshot-style filenames containing a narrow no-break space

Test plan

• pnpm --filter @electric-ax/agents-runtime build
• pnpm --dir packages/agents-server exec vitest run test/electric-agents-routes.test.ts

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.12%. Comparing base (916f6cd) to head (6d67b86).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #4548       +/-   ##
===========================================
+ Coverage   54.80%   74.12%   +19.31%     
===========================================
  Files         317       52      -265     
  Lines       36681     6895    -29786     
  Branches    10466     2189     -8277     
===========================================
- Hits        20104     5111    -14993     
+ Misses      16544     1768    -14776     
+ Partials       33       16       -17     

Flag	Coverage Δ
packages/agents	?
packages/agents-mobile	?
packages/agents-runtime	?
packages/agents-server	74.12% <100.00%> (+0.17%)	⬆️
packages/agents-server-ui	?
packages/electric-ax	?
typescript	74.12% <100.00%> (+19.31%)	⬆️
unit-tests	74.12% <100.00%> (+19.31%)	⬆️

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