Fail fast on unknown model tool calls

[gold-silver-copper]

Summary

• fail fast when models emit tool calls for names Rig did not advertise
• apply the same unknown-tool validation to streaming and non-streaming prompt loops
• surface Gemini malformed-function-call terminal finish reasons as provider errors instead of empty assistant responses
• allow agent-level hooks to be added after tools in the builder chain

Verification

• cargo fmt
• git diff --check
• cargo test -p rig-core prompt_request -- --nocapture
• cargo test -p rig-core malformed_function_call_without_content_is_error -- --nocapture
• cargo check -p rig-core