Upgrade Aspire samples to 13.4

[IEvangelist]

Summary

• Upgrade Aspire SDK/package pins across C# samples, single-file AppHosts, TypeScript aspire.config.json files, and integration tests to 13.4.0.
• Migrate TypeScript AppHosts from legacy apphost.ts / .modules to apphost.mts / .aspire/modules with .mjs imports.
• Update TypeScript AppHost build/CI discovery and remove obsolete dashboard MCP environment variables from checked-in run settings.

Validation

• git diff --check
• Parsed changed JSON files with ConvertFrom-Json
• dotnet msbuild .\build\Build.proj -t:RestoreSample -p:SamplePath=...\samples\python-script\apphost.mts -p:SkipTypeScriptRestore=true -nologo -v:minimal
• aspire restore --apphost apphost.mts --non-interactive and npm run aspire:build in samples\python-script

Release-gated

• dotnet restore .\samples\aspire-shop\AspireShop.AppHost\AspireShop.AppHost.csproj --nologo is expected to fail until Aspire 13.4 packages publish publicly: NuGet currently reports nearest Aspire.AppHost.Sdk version 13.3.5.