refactor(config): replace get_default_provider/get_provider_model with unified get_session_config

[tusharmath]

Summary

Replace the dual get_default_provider / get_provider_model API with a single unified get_session_config method that returns a ModelConfig (provider + model pair) or None.

Context

The previous AppConfigService trait exposed two separate methods — get_default_provider() returning Result<ProviderId> and get_provider_model(Option<&ProviderId>) returning Result<ModelId> — that callers always had to invoke together and chain manually. This split made call-sites verbose, error-prone, and hard to reason about. Since provider and model are always configured atomically via SetSessionConfig, there is no reason to read them separately.

Changes

• Removed get_default_provider and get_provider_model from AppConfigService and replaced them with get_session_config() -> Option<ModelConfig>
• Updated ForgeAppConfigService implementation: the new method reads ForgeConfig::session once and returns Some(ModelConfig) or None, eliminating the intermediate provider_id lookup step
• Updated all call-sites across forge_api, forge_app (agent, agent_provider_resolver, command_generator, data_gen), and forge_main (ui, vscode) to use the new method
• Simplified vscode.rs test helpers by removing the mutex-guarded EnvGuard boilerplate and replacing it with a lighter inline unsafe env-var setup/teardown
• Updated all related tests to assert on the full ModelConfig object instead of provider and model separately

Testing

cargo insta test --accept