#20614: docs(bedrock): add cross-region inference prefix documentation

## Summary Adds documentation explaining AWS Bedrock cross-region inference prefixes (`us.`, `eu.`, `ap.`) and when to use them. ## Changes - Added "Cross-Region Inference" section to `docs/providers/bedrock.md` - Explains prefix usage and when they're needed - Adds troubleshooting section for "model not found" errors ## Issue Fixes #20507 ## Testing - Verified docs render correctly - Checked prefix table formatting <!-- greptile_comment --> <h3>Greptile Summary</h3> Added documentation for AWS Bedrock cross-region inference prefixes (`us.`, `eu.`, `ap.`). Explains when prefixes are needed and provides a clear table with examples for different regions, plus troubleshooting tips for "model not found" errors. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk - Documentation-only change that adds helpful clarification about AWS Bedrock cross-region inference. No code changes, clear formatting, and accurate technical information - No files require special attention _{Last reviewed commit: 16c978d} <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->