show warning when subagents are used but not enabled

[aeschli]

No description provided.

[Copilot]

Pull request overview

This PR adds configuration-aware validation for custom agent subagent usage and extends tests to cover the new behavior.

Changes:

• Injects IConfigurationService into PromptValidator and adds checks so infer and agents headers emit warnings when they configure subagent behavior but chat.customAgentInSubagent.enabled is disabled.
• Introduces an isCustomAgentInSubagentEnabled helper and uses it in validateInfer and validateAgentsAttribute.
• Updates and adds tests in promptValidator.test.ts to assert both the new warning behavior and the config preconditions required for existing infer and agents validations.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/vs/workbench/contrib/chat/common/promptSyntax/languageProviders/promptValidator.ts	Uses IConfigurationService to gate infer and agents validations on ChatConfiguration.SubagentToolCustomAgents, emitting localized warnings when subagent-related settings are present but the feature flag is off.
src/vs/workbench/contrib/chat/test/browser/promptSyntax/languageProviders/promptValidator.test.ts	Aligns the test setup with a shared TestConfigurationService instance and adds targeted tests verifying the new warning behavior and that other combinations remain valid/non-warning as intended.