Add support for custom chat agents in the API (#298227)

* Add support for custom chat agents in the API

- Introduced `chatCustomAgents` proposal in extensions API.
- Implemented methods to handle custom agents in `MainThreadChatAgents2`.
- Added `ICustomAgentDto` interface and related functionality in extHost.
- Created new type definitions for custom agents in `vscode.proposed.chatCustomAgents.d.ts`.

* Filter custom agents by visibility before pushing to the proxy

* Refactor onDidChangeCustomAgents to use direct event listener

* Update custom agent tools property to allow undefined values

* Add chatCustomAgents to enabledApiProposals in package.json

* update

* update

* support skills

* support instructions

* update

* update

---------

Co-authored-by: Martin Aeschlimann <martinae@microsoft.com>

extensions/vscode-api-tests/package.json

@@ -9,6 +9,7 @@
     "authSession",
     "environmentPower",
     "chatParticipantPrivate",
+    "chatPromptFiles",
     "chatProvider",
     "contribStatusBarItems",
     "contribViewsRemote",