Teach MCP-aware prompts about task tokens

[Copilot]

The MCP/tooling prompts did not describe the new task-token model, so the assistant could misinterpret task-scoped credentials and claim they could be revoked directly. This update teaches the prompt stack how task tokens behave and fixes a coupled MCP tool-call handling bug in the same path.

• Prompt behavior

  • Added shared task-token guidance to ChatViewModel for MCP-enabled sessions.
  • Injected that guidance into:
    • agent planning prompts
    • agent execution prompts
    • the main conversation system prompt
  • Guidance now makes the model aware that task tokens:
    • are valid only for the current task
    • grant only the minimum permissions needed
    • remain active until the user marks the task complete
    • cannot be revoked by the assistant

• MCP execution path

  • Fixed ChatViewModel to unwrap Result<String> from McpClientService.callTool(...) before building MultiTurnToolResult.
  • This keeps MCP tool execution aligned with the rest of the multi-turn tool pipeline.

• Focused regression coverage

  • Added prompt-level unit tests to lock in:
    • task-token guidance only appearing for MCP-aware prompts
    • planning prompts carrying the task-token semantics
    • non-MCP prompts remaining unchanged

• Coupled compile fix

  • Resolved an existing AnimatedVisibility compile issue in McpStoreScreen encountered while validating the MCP/chat prompt path.

Example of the prompt guidance now injected for MCP-enabled flows:

internal fun buildTaskTokenGuidance(hasMcpTools: Boolean): String {
    if (!hasMcpTools) return ""
    return """
        Task tokens: Some MCP services use task-scoped tokens.
        Treat them as valid credentials for the current task only.
        They only grant the minimum permissions needed for that task
        and remain active until the user marks the task complete.
        You cannot revoke them yourself.
    """.trimIndent()
}

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