Replace thinking with native Gemini thinking summaries (#400)

This uses Gemini's native [thinking
summaries](https://cloud.google.com/vertex-ai/generative-ai/docs/thinking#thought-summaries)
which were recently added to the API.

Why? The grafted thinking would sometimes cause weird issues where the
model, especially Gemini 2.5 Flash, got confused and put dyad tags like
`<dyad-write>` inside the `<think>` tags.

This also improves the UX because you can see the native thoughts rather
than having the Gemini response load for a while without any feedback.

I tried adding Anthropic extended thinking, however it requires temp to
be set at 1, which isn't ideal for Dyad's use case where we need precise
syntax following.

testing/fake-llm-server/index.ts

@@ -35,17 +35,6 @@ export function createStreamChunk(
 }
 
 export const CANNED_MESSAGE = `
-  <think>
-  \`<dyad-write>\`:
-  I'll think about the problem and write a bug report.
-
-  <dyad-write>
-
-  <dyad-write path="file1.txt">
-  Fake dyad write
-  </dyad-write>
-  </think>
-  
   <dyad-write path="file1.txt">
   A file (2)
   </dyad-write>