Refactor Google and Mistral reasoning stream transitions #50

Summary

Follow up on #47 by aligning the google-genai and mistral chat adapters with the same explicit reasoning-transition style used in the OpenAI adapter.

Motivation

Both adapters currently use a local closeReasoning() helper that mutates reducer state and conditionally returns an event. The OpenAI cleanup in #47 moved away from that pattern in favor of explicit state transitions and functional updates, which makes the reducer easier to follow.

Scope

• refactor packages/google-genai/src/chat-adapter.ts
• refactor packages/mistral/src/chat-adapter.ts
• keep behavior unchanged
• prefer local provider helpers unless a genuinely shared abstraction emerges across providers

Notes

Anthropic likely does not fit the same abstraction because its reasoning lifecycle is driven by explicit block start/stop events.