Fix assistant history text block merging

[KyleAMathews]

Summary

• merge adjacent reconstructed assistant text messages into the previous text block instead of adding multiple text blocks
• keep assistant text + tool call grouping intact
• add a regression test for adjacent assistant text reconstruction

Testing

• pnpm --dir packages/agents-runtime test test/pi-adapter.test.ts

This avoids OpenAI Responses duplicate fallback item ids like msg_153 when replayed assistant history contains multiple unsigned text blocks in a single assistant message.

[github-actions]

Electric Agents Desktop Builds

Build artifacts for commit ba47e61.

Platform	Status	Artifact
macOS Apple Silicon	Passed	DMG
macOS Intel	Passed	DMG
Windows x64	Passed	Installer
Linux x64	Passed	AppImage / deb

Workflow run

[github-actions]

Electric Agents Mobile Build

Android preview build for commit ba47e61.

Platform	Profile	Status	Build
Android	preview	Passed	EAS build

Workflow run

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.23%. Comparing base (b852526) to head (ba47e61).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@                      Coverage Diff                       @@
##           horton/run-failure-details    #4449      +/-   ##
==============================================================
+ Coverage                       56.22%   56.23%   +0.01%     
==============================================================
  Files                             266      266              
  Lines                           28763    28762       -1     
  Branches                         8054     8058       +4     
==============================================================
+ Hits                            16171    16174       +3     
+ Misses                          12577    12573       -4     
  Partials                           15       15              

Flag	Coverage Δ
packages/agents	70.37% <ø> (ø)
packages/agents-mobile	85.41% <ø> (ø)
packages/agents-runtime	81.88% <100.00%> (+0.03%)	⬆️
packages/agents-server	75.14% <ø> (ø)
packages/agents-server-ui	5.69% <ø> (ø)
packages/electric-ax	46.33% <ø> (ø)
typescript	56.23% <100.00%> (+0.01%)	⬆️
unit-tests	56.23% <100.00%> (+0.01%)	⬆️

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