Avoid starting web host in MCP stdio mode

[jstar0]

Summary

dab start --mcp-stdio communicates over stdin/stdout, but the stdio helper was calling host.Start(). That starts the web host and Kestrel, so stdio mode still tried to bind the configured HTTP port. When the port was occupied, the process exited before the MCP initialize handshake.

The stdio path already initializes and registers MCP tools directly from DI, so it can run the stdio JSON-RPC loop without starting Kestrel.

Fixes #3675.

Changes

• Avoid starting the ASP.NET Core host when running the engine in MCP stdio mode.
• Keep MCP tool registry initialization in the stdio path before running the JSON-RPC loop.
• Dispose the host after the stdio loop exits without calling StartAsync() or StopAsync().
• Add a regression test that proves RunMcpStdioHost() does not call IHost.StartAsync()/StopAsync(), still starts the stdio server loop, and disposes the host.

Verification

dotnet test src/Service.Tests/Azure.DataApiBuilder.Service.Tests.csproj --filter 'FullyQualifiedName~McpStdioHelperTests' -p:RuntimeIdentifiers= -p:UseAppHost=false
dotnet test src/Service.Tests/Azure.DataApiBuilder.Service.Tests.csproj --filter 'FullyQualifiedName~McpStdioHelperTests|FullyQualifiedName~McpStdioServerRunAsyncTests|FullyQualifiedName~McpStdioServerInitializeTests|FullyQualifiedName~McpStdioServerContentBlockTests|FullyQualifiedName~McpToolRegistryTests' -p:RuntimeIdentifiers= -p:UseAppHost=false
git diff --check

A runtime smoke also occupied 127.0.0.1:5155, pointed ASPNETCORE_URLS at that occupied port, and verified that the service completed MCP initialize and shutdown over stdio without an address-in-use error.

[Copilot]

Pull request overview

This PR fixes MCP stdio mode (dab start --mcp-stdio) so it no longer starts the ASP.NET Core web host (and therefore doesn’t attempt to bind HTTP ports via Kestrel) before completing the MCP initialize handshake.

Changes:

• Remove the IHost.Start() / StopAsync() lifecycle calls from the MCP stdio helper path.
• Keep MCP tool discovery/registration via DI, then run the stdio JSON-RPC loop.
• Add a regression unit test asserting stdio mode does not start the host while still running the stdio server loop.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/Service/Utilities/McpStdioHelper.cs	Stops starting/stopping the host in MCP stdio mode; runs tool registry init + stdio loop without binding HTTP ports.
src/Service.Tests/UnitTests/McpStdioHelperTests.cs	Adds a regression test validating stdio mode doesn’t start the web host while still running the stdio server loop.

[JerryNixon]

Clean and simple. Thank you for the PR. Looks great. Small tweaks.

[RubenCerna2079]

LGTM! Just need you to address the comments made by copilot. If you think they are not necessary, just give your reasoning and resolve the issue.

[RubenCerna2079]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 6 pipeline(s).

[RubenCerna2079]

@jstar0 this PR is ready to merge in. Thanks for your work!