[STG-1927] Add local MCP model gateway TDD coverage

[monadoid]

Summary

Adds failing TDD coverage for local Browserbase MCP model-gateway behavior.

The tests describe the expected behavior before implementation:

• config accepts provider/model modelName without modelApiKey
• config resolution preserves an omitted model key instead of injecting a dummy provider key
• Stagehand construction uses Browserbase model gateway when only a Browserbase API key and model name are provided
• explicit provider modelApiKey remains the highest-priority BYOK path

Validation

• pnpm test runs and fails on the new intended assertions:
  • schema still requires modelApiKey for custom models
  • config still injects dummy-api-key
  • Stagehand construction still passes the raw model string instead of gateway-backed model config
• pnpm install and commit hooks completed; unrelated lockfile formatting from install was reverted.