[ci] chore: add gemini code assistant config (#2349)

### What does this PR do?

add gemini code assistant config. The example code reviews are in
https://github.com/eric-haibin-lin/verl/pull/17#pullrequestreview-2977403886.
The threshold is set to high to avoid too many review comments.

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.gemini/config.yaml

@@ -0,0 +1,10 @@
+have_fun: false
+code_review:
+  disable: false
+  comment_severity_threshold: HIGH
+  max_review_comments: -1
+  pull_request_opened:
+    help: false
+    summary: false
+    code_review: true
+ignore_patterns: []