fix(e2e): retain local dev output on failure

[Hweinstock]

Problem

Follow-up to #2394. That PR added logging of the last message written to stdout/stderr, but this is not enough information.

Solution

This PR dumps the entire output into the error.

Verification

Ran tests locally with a failure in dev and verified the output was there in local logs.

 × e2eTest/project/templates.test.ts > add, dev, deploy, invoke for runtime templates > local invocation > 'agent_python_minimal' runs locally 5006ms
   → agentcore dev exited with code 1.  
stdout/stderr = INJECTED
Error: FAILURE

 × e2eTest/project/templates.test.ts > add, dev, deploy, invoke for runtime templates > local invocation > 'agent_python_strands' runs locally 5006ms
   → agentcore dev exited with code 1.  
stdout/stderr = INJECTED
Error: FAILURE

 × e2eTest/project/templates.test.ts > add, dev, deploy, invoke for runtime templates > local invocation > 'py_strands_container' runs locally 5007ms
   → agentcore dev exited with code 1.  
stdout/stderr = INJECTED
Error: FAILURE

 × e2eTest/project/templates.test.ts > add, dev, deploy, invoke for runtime templates > local invocation > 'agent_python_langchain' runs locally 5007ms
   → agentcore dev exited with code 1.  
stdout/stderr = INJECTED
Error: FAILURE

 × e2eTest/project/templates.test.ts > add, dev, deploy, invoke for runtime templates > local invocation > 'agent_ts_strands' runs locally 5008ms
   → agentcore dev exited with code 1.  
stdout/stderr = INJECTED
Error: FAILURE

after adding:

 console.log("INJECTED");
 throw new Error("FAILURE");

into the dev handler.

[agentcore-devx-automation]

AgentCore Harness Review

Verdict: Looks good

Small, well-targeted test-only diagnostic improvement. The previous error messages interpolated pendingOutput, which only ever contains the trailing partial line (everything up to the last newline is consumed by lines = pendingOutput.split(...); pendingOutput = lines.pop()), so failure messages were nearly always empty. Accumulating a separate devOutput and using that in errors — plus fixing the stdout/stdout → stdout/stderr typo — makes flaky failures much easier to debug. devOutput growing unbounded is fine for a short-lived e2e test process. No new production code, so telemetry/mocking concerns don't apply. LGTM.

[agentcore-devx-automation]

Claude Security Review: no high-confidence findings. (run)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.25%. Comparing base (5250b42) to head (0cbfc4c).

Additional details and impacted files

@@            Coverage Diff            @@
##           refactor    #2398   +/-   ##
=========================================
  Coverage     97.25%   97.25%           
=========================================
  Files           612      612           
  Lines         41004    41004           
=========================================
  Hits          39878    39878           
  Misses         1126     1126           

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