fix(harness): import fi.simulate on the callback adapter's main thread

[KarthikAvinashFI]

Why

Every hosted chat run of a callback-style agent fails before the first turn with target_agent_failed: Plugins must be registered on the main thread.

The callback adapter imports fi.simulate inside _invoke, which runs on a ThreadingHTTPServer worker thread. With livekit installed, as in the hosted runtime image, that import registers livekit plugins, and livekit refuses registration off the main thread. Voice agents do not use this adapter and are unaffected.

What changed

File	Change
src/fi/alk/harness/callback_http_adapter.py	from fi.simulate import AgentInput moves to module level, so it runs on the main thread at process start.
tests/harness/test_callback_http_adapter.py	Runs the adapter as a bundle does (its source copied out of the package) and sends one /invoke. Fails before the fix when livekit is installed; passes after.

Testing

• New test: fails on dev and passes on this branch, in an environment with the livekit extra.
• tests/harness -k "bundle_author or callback or chat": 114 passed.
• ruff check and ruff format --check clean.

[KarthikAvinashFI]

Folded into #128.