Adopt latest provider pattern for org/enterprise custom agents

[pwang347]

See microsoft/vscode#280426

[Copilot]

Pull request overview

This PR adopts the latest provider pattern for org/enterprise custom agents, migrating from the old CustomAgentsProvider API to the new provider-based approach introduced in VS Code core (see microsoft/vscode#280426).

Changes:

• Replaced CustomAgentsProvider interface with new CustomAgentProvider, InstructionsProvider, and PromptFileProvider interfaces
• Updated API registration from registerCustomAgentsProvider to registerCustomAgentProvider
• Changed return types from interface-based CustomAgentResource objects to class-based CustomAgentChatResource instances
• Made isExternalInstructionsFile method async to support checking extension-contributed prompt files

Reviewed changes

Copilot reviewed 11 out of 13 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/vscodeTypes.ts	Added export for new CustomAgentChatResource type
src/util/common/test/shims/vscodeTypesShim.ts	Added CustomAgentChatResource to the VS Code types shim
src/util/common/test/shims/chatTypes.ts	Implemented CustomAgentChatResource class for testing
src/platform/test/common/testCustomInstructionsService.ts	Updated mock to return Promise from isExternalInstructionsFile
src/platform/customInstructions/common/customInstructionsService.ts	Made isExternalInstructionsFile async and added logic to check extension-contributed prompt files via command
src/extension/vscode.proposed.chatPromptFiles.d.ts	Added new proposed API definitions for chat resource classes and providers
src/extension/vscode.proposed.chatParticipantPrivate.d.ts	Removed old CustomAgentResource and CustomAgentsProvider definitions
src/extension/tools/node/toolUtils.ts	Added await for now-async isExternalInstructionsFile call
src/extension/extension/vscode-node/contributions.ts	Renamed contribution class from OrganizationAndEnterpriseAgentContribution to PromptFileContribution
src/extension/agents/vscode-node/test/organizationAndEnterpriseAgentProvider.spec.ts	Updated tests to work with new resource class pattern and fixed timing issues with background fetch
src/extension/agents/vscode-node/promptFileContrib.ts	Renamed class and updated to use new registerCustomAgentProvider API
src/extension/agents/vscode-node/organizationAndEnterpriseAgentProvider.ts	Updated provider to implement new interface and return CustomAgentChatResource instances
package.json	Updated activation event and enabled chatPromptFiles proposed API