Add Claude Opus 4.8 support (#57984)

<img width="627" height="752" alt="Screenshot 2026-05-28 at 1 20 22 PM"
src="https://github.com/user-attachments/assets/0a7825f0-73c5-49e9-b59a-83924a45de98"
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Adds Claude Opus 4.8 for BYOK providers, including Anthropic fast-mode
handling and Bedrock/OpenCode model definitions.

Closes AI-336

Release Notes:

- Added Claude Opus 4.8 BYOK support
a6b0ee9f36Richard Feldman committed on 5/28/2026, 5:45:54 PM· committed by GitHubparent3bb2f2a
7 files changedLine totals unavailable