getProviderLabel was string-matching `anthropic.claude-` in model IDs,
which false-positived on cross-region IDs like `us.anthropic.claude-sonnet-4-6`.
Now checks `CLAUDE_CODE_USE_BEDROCK=1` env var instead.
Fixes#466
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>