fix(messages): add Anthropic count tokens endpoint

[hangox]

Summary

• add POST /v1/messages/count_tokens for Anthropic-compatible clients
• validate the count-tokens request shape and return { input_tokens }
• keep the endpoint local-only so it does not require an authenticated Codex account or an upstream request
• reuse the configured proxy API key check when server.proxy_api_key is set

Why

Claude Code calls Anthropic SDK beta.messages.countTokens(...), which maps to /v1/messages/count_tokens?beta=true. Without this endpoint, /context token accounting can fall back to inaccurate estimates and significantly over-report categories such as custom agents.

Testing

• npm test -- tests/e2e/messages.test.ts tests/unit/routes/messages-count-tokens.test.ts
• npx tsc --noEmit
• npm run build