Map Claude opus 1m alias for Claude Code

[Icewind52]

Summary

• map Anthropic model alias claude-opus-4-6-1m to Copilot model id claude-opus-4.6-1m
• keep alias handling in the Claude messages translation path used by Claude Code
• make /v1/messages/count_tokens resolve the translated model id for accurate token counting
• add a regression test for the new alias mapping

Testing

• bun test tests/anthropic-request.test.ts