[FEATURE] Add Claude Opus 4.8 (anthropic.claude-opus-4-8) to the model list

[NitroBoss]

What

Claude Opus 4.8 is live in Amazon Bedrock (anthropic.claude-opus-4-8, lifecycle ACTIVE), but bedrock-chat's model map currently tops out at claude-v4.7-opus. Adding it follows the same pattern as the Opus 4.7 addition (#1101): same request surface (adaptive thinking only, no assistant prefill, no temperature/top_p/top_k) and the same list pricing as 4.7 ($5/$25 per MTok, no long-context premium).

Verified (live, from an eu-central-2 deployment)

• aws bedrock list-foundation-models --region eu-central-2 → anthropic.claude-opus-4-8 ACTIVE
• aws bedrock list-inference-profiles --region eu-central-2 → eu.anthropic.claude-opus-4-8 and global.anthropic.claude-opus-4-8 ACTIVE
• get-inference-profile on the eu. profile → covers eu-central-1, eu-central-2, eu-north-1, eu-south-1, eu-south-2, eu-west-1, eu-west-3. Note: eu-west-2 is NOT covered (it is for 4.7), so copying the 4.7 region list would be wrong.

Caveat

Our AWS org restricts us to EU regions, so the us/jp/au/apac entries of the inference maps can't be verified from here. The PR I'm submitting keeps those geos out and ships only the verified EU + global entries; maintainers can extend the maps by running scripts/cross_region_inference/get_supported_cross_region_inferences.py from an unrestricted account.

PR follows, mirroring #1101 file-by-file.