fix(anthropic): don't double-prefix Bedrock cross-region inference profile model IDs

[aldouus]

Summary

• bedrockPrefixModelWithRegion only skipped prefixing when the model ID already started with the caller region's two-letter prefix. Any other valid Bedrock cross-region inference profile got mangled: global.anthropic.claude-fable-5-1 in us-east-1 became us.global.anthropic.claude-fable-5-1, and apac.anthropic.* in ap-southeast-2 became ap.apac.anthropic.*.
• Now any ID already carrying a known inference profile prefix (global., us., us-gov., eu., apac., ap., jp., au., ca., sa., il., mx.) is passed through untouched; bare anthropic.* IDs still get the region prefix.
• Motivation: Claude Fable 5.1 ships on Bedrock as both us.anthropic.claude-fable-5-1 (Geo CRIS) and global.anthropic.claude-fable-5-1 (Global CRIS). The latter is unusable without this fix.

Test plan

• New TestBedrockPrefixModelWithRegion covers bare IDs, matching geo, cross-geo, global., and apac.
• go test -tags http2legacy ./providers/anthropic/

Note: -tags http2legacy is needed on Go 1.27 because golang.org/x/net/http2 is excluded there (errors.go references http2.GoAwayError). That build failure exists on main independent of this change.