Set Gemini Code Assist include_drafts to false (#10765)

Set `include_drafts` to false so Gemini Code Assist doesn't comment on draft PRs.

"Enables agent functionality on draft pull requests. Default: true."
https://developers.google.com/gemini-code-assist/docs/customize-gemini-behavior-github#config.yaml-schema

Match behavior in flutter/flutter https://github.com/flutter/flutter/pull/175098

## Pre-Review Checklist

**Note**: The Flutter team is currently trialing the use of [Gemini Code Assist for GitHub](https://developers.google.com/gemini-code-assist/docs/review-github-code). Comments from the `gemini-code-assist` bot should not be taken as authoritative feedback from the Flutter team. If you find its comments useful you can update your code accordingly, but if you are unsure or disagree with the feedback, please feel free to wait for a Flutter team member's review for guidance on which automated comments should be addressed.

[^1]: Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling.

.gemini/config.yaml

@@ -11,6 +11,7 @@ code_review:
     # These tend to be verbose, and since we expect PR authors to clearly
     # describe their PRs this would be at best duplicative.
     summary: false
+    include_drafts: false
 ignore_patterns:
   - .ci/flutter_master.version
   - .ci/flutter_stable.version