fix: make the documented demo test command actually run tests

[twcclegg]

Problem

csharp/PhoneNumbers.Demo/AGENTS.md makes running the demo suite a mandatory step, with this command:

dotnet test csharp/PhoneNumbers.Demo.Tests -p:TargetFrameworks=net10.0

That command runs zero tests and exits 0:

A total of 1 test files matched the specified pattern.
No test is available in .../PhoneNumbers.Demo.Tests.dll. Make sure that test discoverer & executors
are registered and platform & framework version settings are appropriate and try again.

exit code: 0

PhoneNumbers.Demo.Tests declares the singular <TargetFramework>net10.0</TargetFramework>, so -p:TargetFrameworks=net10.0 sets a property the project never reads and promotes the build to a cross-targeting outer build, where no tests are discovered. Because dotnet test exits 0 on zero discovery, the result reads as a pass.

The net effect is that anyone — human or agent — following the mandatory workflow gets a green light for a suite that never ran. That is the precise failure the workflow exists to prevent.

Fix

The -p:TargetFrameworks=net10.0 override is vestigial here — it's copy-pasted from the repo-root convention (where PhoneNumbers.Test/PhoneNumbers.Extensions.Test genuinely multi-target net8.0;net10.0 and the switch does real work), but PhoneNumbers.Demo.Tests only ever targets net10.0 and gets nothing from declaring it plural. So the fix removes the override from the one documented command that doesn't need it, rather than reshaping the csproj into permanent cross-targeting form to tolerate a flag that shouldn't be there — that alternative would pay outer/inner cross-targeting build overhead forever for a project that will never multi-target, and reads as "designed to support multiple frameworks" to the next contributor.

AGENTS.md additionally notes that a zero-discovery run still exits 0 regardless, so the count is worth checking either way.

Verification

Command	Before	After
dotnet test csharp/PhoneNumbers.Demo.Tests -p:TargetFrameworks=net10.0	0 tests, exit 0	n/a — no longer the documented command
dotnet test csharp/PhoneNumbers.Demo.Tests	67 passed	67 passed
CI sequence (restore / build --no-restore / test --no-build)	67 passed	67 passed

CI (build_and_run_demo_tests.yml) never passed -p:TargetFrameworks in the first place, so it was never exposed to this bug and isn't affected either way.

Scope note: the repo-wide dotnet test csharp/PhoneNumbers.slnx -p:TargetFrameworks=net10.0 in the root AGENTS.md, README.md and CONTRIBUTING.md is not affected — the demo projects aren't in PhoneNumbers.slnx, and both solution test projects genuinely multi-target, so the switch does real work there.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.68%. Comparing base (8ad7fa2) to head (c403a0d).
⚠️ Report is 14 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #456      +/-   ##
==========================================
+ Coverage   87.51%   87.68%   +0.16%     
==========================================
  Files          43       43              
  Lines        3886     3889       +3     
  Branches      991      992       +1     
==========================================
+ Hits         3401     3410       +9     
+ Misses        280      276       -4     
+ Partials      205      203       -2     

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