fix(core): preserve global model config with custom clients - #3094

Summary

• preserve global model configuration when createOpenAIClient is provided without an explicit modelConfig
• expose the agent-scoped client factory through agent.modelConfigManager, including Playground describe and locate runtimes
• keep custom client factories isolated between agents that share global model values

Root cause

Agent treated createOpenAIClient alone as a custom model configuration and created an isolated ModelConfigManager. Because that manager had neither an explicit modelConfig nor a registered GlobalConfigManager, it resolved an empty configuration and reported that MIDSCENE_MODEL_NAME was missing.

Simply reusing the global manager would fix model resolution but would drop the agent-scoped factory from public modelConfigManager consumers. Playground builds describe and locate runtimes through that public manager, so observability wrappers such as LangSmith or Langfuse would be silently skipped on those paths.

Solution

Factory-only agents now use an AgentScopedModelConfigManager that delegates model values to globalModelConfigManager and returns a copied configuration with the current agent's createOpenAIClient.

This keeps global overrides and environment updates dynamic, avoids mutating shared cached configurations, and preserves independent factories for multiple agents.

Tests use stubbed environment variables with explicit cache clearing so global configuration state is restored after every case.

Validation

• pnpm --filter @midscene/core test
  • 150 test files passed
  • 1,564 tests passed, 8 skipped
• pnpm build
  • 28 projects built successfully
• pnpm type-check:tests
• pnpm run lint

Closes #3079.