feat(core): support openai/gpt-5.5 in CUA model allowlist

[BABTUNA]

why

openai/gpt-5.5 is now available, but CUA model validation/mapping still only includes openai/gpt-5.4.

This blocks users from selecting openai/gpt-5.5 in CUA mode even though provider behavior is the same family.

proposed change

• Add gpt-5.5 to modelToAgentProviderMap in AgentProvider
• Add openai/gpt-5.5 to AVAILABLE_CUA_MODELS
• Update public API type test list to include openai/gpt-5.5

scope

Small parity update (no behavior changes beyond allowing model selection).

validation

• unit: tests/unit/public-api/llm-and-agents.test.ts