test: add app-server auto environment helper

[anp-oai]

Why

Start moving towards app-server tests defaulting to running against remote & foreign OS executors. To do so we need a point of indirection similar to core integration tests' build_with_auto_env, but with the flexibility of letting tests control environment registration if they need to.

What

This adds:

• TestAppServer::new_with_auto_env() for constructing an app server with a default environment defined by the test runner (e.g. bazel)
• TestAppServer::auto_env_params() for tests to easily acquire turn env params tailored to the automatic environment
• TestAppServer::send_thread_start_request_with_auto_env() to make it easy for tests to start a thread using the automatic environment

The above methods all fail if the test calling them has set up an environment where the automatic environment configuration conflicts with test-created state.

Validation

Adds a couple of basic smoke tests to the app-server test suite. Follow-ups will migrate more tests to use it.

[anp-oai]

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[chatgpt-codex-connector]

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