test(hooks): add test coverage for model-fallback hook

[Hungdoan565]

Add 17 tests for the model-fallback hook's FallbackStateController covering:

• setSessionFallbackChain / getSessionFallbackChain (defensive copy, undefined, empty sessionID)
• clearSessionFallbackChain
• setPendingModelFallback (no chain, empty chain, valid chain, already pending, chain exhausted, agent requirements fallback)
• getNextFallback / clearPendingModelFallback
• reset

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Summary by cubic

Add 17 unit tests for the model-fallback FallbackStateController covering session chain I/O, pending fallback setup, next-fallback retrieval, and reset/cleanup, including undefined/empty chains, agent-derived chains, already-pending/exhausted chains, and defensive copies. Sync bun.lock platform binaries for oh-my-opencode-* to 4.5.1.

^{Written for commit 8c21c83. Summary will update on new commits. Review in cubic}