Add TaskGroup testing example for basic_task_group agent

[kath0la]

Description

Add a reference test file demonstrating how to test TaskGroup-based
agents using FakeLLM and the testing best practices from the docs (there
is a docs PR adding some guidelines for writing tests for task groups
and I'd like to link to an example test.)

Tests cover:

• Each task (CollectNameTask, CollectEmailTask) in isolation
• Sequential TaskGroup flow with onTaskCompleted callbacks
• summarizeChatCtx producing a history summary
• Single, repeated, and cross-task regressions via out_of_scope

Co-Authored-By: Claude Opus 4.6 (1M context) noreply@anthropic.com

Testing

pnpm install
pnpm build
pnpm test examples/src/testing/*

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[kath0la]

@toubatbrian If there's another test I can refer to or update, let me know. I wanted to link to a test for an agent that we can also link to as an example usage of TaskGroup.

[devin-ai-integration]

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Devin Review analyzed this PR and found no potential bugs to report.

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[toubatbrian]

lg!

[toubatbrian]

@toubatbrian If there's another test I can refer to or update, let me know. I wanted to link to a test for an agent that we can also link to as an example usage of TaskGroup.

I think this one is pretty good tho, what specific format are you expecting to have?

[kath0la]

@toubatbrian If there's another test I can refer to or update, let me know. I wanted to link to a test for an agent that we can also link to as an example usage of TaskGroup.

I think this one is pretty good tho, what specific format are you expecting to have?

@toubatbrian If you think this is good, then I'll just merge it! I wanted an example that was well documented in code, and that demonstrated the best practices mentioned in the docs. Thank you!