fix: support hyphenated Claude model names for Claude Code 2.1.x compatibility

[yang-builds]

Problem

Claude Code 2.1.x uses hyphenated model names (e.g. claude-sonnet-4-6) for its internal display mapping, but the GitHub Copilot backend only accepts dot notation (e.g. claude-sonnet-4.6).

The current translateModelName truncates claude-sonnet-4-6 to claude-sonnet-4, which causes:

1. GitHub Copilot backend rejects the request with HTTP 400
2. Claude Code welcome screen shows "Sonnet 4" instead of "Sonnet 4.6"

Root Cause

// Before — incorrect truncation
if (model.startsWith("claude-sonnet-4-")) {
  return model.replace(/^claude-sonnet-4-.*/, "claude-sonnet-4")  // ❌ drops minor version
}

claude-sonnet-4-6 → claude-sonnet-4 → rejected by Copilot backend

Fix

Replace with a regex that correctly converts hyphenated minor version to dot notation:

// After — correct conversion
const m = model.match(/^(claude-(?:sonnet|opus|haiku))-(\d+)-(\d+)$/)
if (m) return `${m[1]}-${m[2]}.${m[3]}`

Input	Output	Result
claude-sonnet-4-6	claude-sonnet-4.6	✅ HTTP 200
claude-opus-4-7	claude-opus-4.7	✅ HTTP 200
claude-haiku-4-5	claude-haiku-4.5	✅ HTTP 200
claude-sonnet-4.6	claude-sonnet-4.6	✅ passthrough (unchanged)

Future models (e.g. claude-sonnet-5-0) are automatically handled without any code changes.