fix: retrieve stored result from tasks/result for failed tasks

[Christian-Sidak]

Summary

When a task-augmented request reaches failed terminal status, TaskManager.requestStream was yielding a hardcoded ProtocolError(InternalError, "Task <id> failed") instead of calling tasks/result to retrieve the actual stored result. This meant whatever the handler stored (e.g. { isError: true, content: [...] }) was unreachable by the client.

The fix mirrors the completed case: call this.getTaskResult(...) and yield { type: 'result', result }, matching the spec requirement that tasks/result must return exactly what the underlying request would have returned.

Root cause (packages/core/src/shared/taskManager.ts:310-312):

// Before (wrong)
case 'failed': {
    yield { type: 'error', error: new ProtocolError(ProtocolErrorCode.InternalError, `Task ${taskId} failed`) };
    break;
}

// After (correct, mirrors 'completed' branch)
case 'failed': {
    const result = await this.getTaskResult({ taskId }, resultSchema, options);
    yield { type: 'result', result };
    break;
}

Test plan

• Added integration test in test/integration/test/taskLifecycle.test.ts that:
  • Calls callToolStream on a tool registered with shouldFail: true
  • Asserts the terminal stream event is { type: 'result' } (not { type: 'error' })
  • Asserts result.isError === true and result.content matches the stored failure text
• All 489 core unit tests pass
• All integration task lifecycle tests pass

Fixes #1922

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Latest commit: 1e82e88

The changes in this PR will be included in the next version bump.

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Name	Type
@modelcontextprotocol/core	Patch

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