bug(core): Ensure we use thinking budget on fallback to 2.5

[joshualitt]

Summary

Fixes #13567 . Today, when we fallback to 2.5 with a 3 config, we have a thinkingLevel property we need to unset.

[github-actions]

Size Change: +219 B (0%)

Total Size: 21.1 MB

ℹ️ View Unchanged

Filename	Size	Change
./bundle/gemini.js	21.1 MB	+219 B (0%)
./bundle/sandbox-macos-permissive-closed.sb	1.03 kB	0 B
./bundle/sandbox-macos-permissive-open.sb	890 B	0 B
./bundle/sandbox-macos-permissive-proxied.sb	1.31 kB	0 B
./bundle/sandbox-macos-restrictive-closed.sb	3.29 kB	0 B
./bundle/sandbox-macos-restrictive-open.sb	3.36 kB	0 B
./bundle/sandbox-macos-restrictive-proxied.sb	3.56 kB	0 B

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[SandyTao520]

/patch stable

[github-actions]

✅ Patch workflow(s) dispatched successfully!

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• Channels: stable
• Commit: fe67ef63c11a8b1a591e0767789dedc56c8d1ce2
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🚀 Patch PR Created!

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• Environment: prod
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• Commit: fe67ef63c11a8b1a591e0767789dedc56c8d1ce2
• Hotfix Branch: hotfix/v0.18.1/0.18.2/stable/cherry-pick-fe67ef6/pr-13596
• Hotfix PR: #13862

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✅ Patch Release Complete!

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• Version: 0.18.2
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