Enhance CopilotSlashCommandCompletionProvider with session customizations and improve command handling

[DonJayamanne]

No description provided.

[Copilot]

Pull request overview

This PR enhances Copilot’s slash-command completion flow in the agent host by incorporating session customizations (to suppress duplicate skill suggestions) and by adding runtime skill handling (including mid-message / token completions). It also adjusts runtime command dispatch/listing so skills are available from the runtime catalog and aren’t incorrectly invoked as regular commands.

Changes:

• Extend CopilotSlashCommandCompletionProvider to (a) support whitespace-delimited slash tokens for skill-only completions and (b) filter runtime skill completions based on session-known skills/customizations.
• Update runtime command handling so skills are included in the runtime command catalog and are not invoked through the commands.invoke path.
• Add extensive unit tests covering runtime skill completion behaviors and deduplication against session customizations.

Show a summary per file

File	Description
src/vs/platform/agentHost/test/node/copilotSlashCommandCompletionProvider.test.ts	Adds test coverage for runtime skill completions and filtering against known (customization-provided) skills.
src/vs/platform/agentHost/node/copilot/copilotSlashCommandCompletionProvider.ts	Implements session-customization-aware runtime skill completion, skill hint handling, and in-message slash token behavior.
src/vs/platform/agentHost/node/copilot/copilotAgentSession.ts	Includes runtime skills in the command list cache and avoids invoking skills via the command-invoke RPC path.
src/vs/platform/agentHost/node/copilot/copilotAgent.ts	Wires session customizations into the completion provider so it can compute “known skills” per session.

Review details

• Files reviewed: 4/4 changed files
• Comments generated: 3
• Review effort level: Low