chore(deps)!: migrate from @google/generative-ai to @google/genai

The @google/generative-ai SDK has been deprecated by Google. This switches
to the successor @google/genai package and rewrites the Gemini provider
against the new stateless models/chats API.

BREAKING CHANGE: GeminiConfig.safetySettings now uses the SafetySetting
type from @google/genai. Consumers passing this field must update their
import from '@google/generative-ai' to '@google/genai'. The shape of the
type is similar but not identical.

Notable simplifications enabled by the new SDK:
- No per-model client caching: generateContent specifies model per-call
- Single code path for both single- and multi-turn (full contents array
  is passed to generateContent directly; no more startChat branching)
- Stream chunks expose .text as a property (was .text() method)
- Stream iteration is direct on the response (no .stream sub-property)

Default model bumped from gemini-1.5-flash to gemini-2.5-flash.

package.json

@@ -53,7 +53,7 @@
   },
   "dependencies": {
     "@anthropic-ai/sdk": "^0.97.0",
-    "@google/generative-ai": "^0.24.1",
+    "@google/genai": "^2.5.0",
     "openai": "^6.0.0"
   },
   "devDependencies": {