feat: add DevTools debugging UI for core branch

[rajan-chari]

Summary

• Ports the DevTools browser-based debugging UI from main's plugin system to the upstream/next/core middleware architecture
• New Microsoft.Teams.Bot.DevTools project under core/src/ — provides real-time WebSocket activity monitoring, test message injection, and app metadata display at /devtools
• Uses ITurnMiddleware for incoming activity interception, ConversationClient override for outgoing, and minimal API endpoints (replacing MVC controllers)

Architecture

Component	Role
DevToolsMiddleware	ITurnMiddleware — emits received/error events
DevToolsConversationClient	Extends ConversationClient — emits sent events
DevToolsService	Singleton shared state: WebSocket connections, metadata
DevToolsHostingExtensions	AddDevTools() + UseDevTools() registration
web/	Embedded React UI (copied from main, unchanged)

Usage

builder.Services.AddTeamsBotApplication();
builder.Services.AddDevTools();          // NEW

var app = builder.Build();
app.UseTeamsBotApplication();
app.UseDevTools();                       // NEW

Verification

• Builds with 0 warnings, 0 errors on both net8.0 and net10.0
• All 447 existing tests pass
• Full solution builds cleanly

Test plan

• Build: dotnet build core/src/Microsoft.Teams.Bot.DevTools/
• Run existing tests: dotnet test core/core.slnx
• Add DevTools to a sample bot and verify /devtools UI loads
• Verify WebSocket connection receives activity events
• Verify test message injection via DevTools UI
• Verify outgoing activities appear in DevTools

🤖 Generated with Claude Code

[rido-min]

closing, superseed by #381