test(runtimes/dotnet): add unit and integration tests for dotnet runtime

[Copilot]

Test-gap analysis flagged the runtimes/dotnet module as having 0 direct tests for generate_dotnet_install / generate_ensure_nuget_config and no end-to-end compile coverage. Note: most DotnetExtension::validate() branches called out in the issue are already covered in src/compile/extensions/tests.rs, so this PR fills only the genuine gaps.

Summary

• src/runtimes/dotnet/mod.rs — new #[cfg(test)] mod tests block:
  • generate_dotnet_install: default (8.0.x), explicit version, global.json sentinel (emits useGlobalJson: true, drops explicit version), case-insensitive sentinel.
  • generate_ensure_nuget_config: feed URL is interpolated into the heredoc with valid XML; default falls back to https://api.nuget.org/v3/index.json.
• tests/compiler_tests.rs — two E2E tests modeled on test_lean_runtime_compiled_output:
  • test_dotnet_runtime_compiled_output — runtimes: dotnet: true emits UseDotNet@2, default 8.0.x, dotnet in the bash allow-list, and nuget.org in the AWF allow-domains.
  • test_dotnet_runtime_with_feed_url_compiled_output — feed-url: additionally emits the ensure-nuget.config shim and NuGetAuthenticate@1.

No production code changed.

Test plan

• cargo test: 1231 unit (+6) / 82 integration (+2) / 3 init / 8 mcp-http — all pass.
• cargo clippy --all-targets --all-features — clean (only pre-existing warnings).

Warning

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• spsprodeus21.vssps.visualstudio.com
  • Triggering command: /home/REDACTED/work/ado-aw/ado-aw/target/debug/deps/ado_aw-632c4d56de6fcebc /home/REDACTED/work/ado-aw/ado-aw/target/debug/deps/ado_aw-632c4d56de6fcebc --emit=dep-info,metadata -C embed-bitcode=no -C debuginfo=2 --check-cfg /lib.rs --check-cfg cfg(feature, values()) -C metadata=212eb341856bbdc9 know�� known-linux-gnu/--error-format=json known-linux-gnu/--json=diagnostic-rendered-ansi,artifacts,future-incompat bin/rustc known-linux-gnu/cc -incompat known-linux-gnu/-Wl,--no-undefined-version bin/rustc (dns block)
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  • Triggering command: /home/REDACTED/work/ado-aw/ado-aw/target/debug/deps/ado_aw-632c4d56de6fcebc /home/REDACTED/work/ado-aw/ado-aw/target/debug/deps/ado_aw-632c4d56de6fcebc --emit=dep-info,metadata -C embed-bitcode=no -C debuginfo=2 --check-cfg /lib.rs --check-cfg cfg(feature, values()) -C metadata=212eb341856bbdc9 know�� known-linux-gnu/--error-format=json known-linux-gnu/--json=diagnostic-rendered-ansi,artifacts,future-incompat bin/rustc known-linux-gnu/cc -incompat known-linux-gnu/-Wl,--no-undefined-version bin/rustc (dns block)
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