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.

e2e-tests/snapshots/import.spec.ts_import-app-2.aria.yml

@@ -1,12 +1,4 @@
 - paragraph: /Generate an AI_RULES\.md file for this app\. Describe the tech stack in 5-\d+ bullet points and describe clear rules about what libraries to use for what\./
-- 'button "Thinking `＜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>"':
-  - img
-  - img
-  - paragraph:
-    - code: "`＜dyad-write>`"
-    - text: ": I'll think about the problem and write a bug report."
-  - paragraph: ＜dyad-write>
-  - paragraph: ＜dyad-write path="file1.txt"> Fake dyad write ＜/dyad-write>
 - img
 - text: file1.txt
 - img