test: bring the test assembly to 100% line coverage

[Chris-Wolfgang]

Summary

Steps 2 and 3 of #310. The test assembly was already instrumented (IncludeTestAssembly came in with #343) and measured 97.7%. Every gap was a line that cannot execute while the test passes. Each one is fixed without weakening the test.

Class	Was	Fix
DocExampleCompilationTests	90.6%	if (errors.Count > 0) Assert.Fail(Format(...)) → Assert.True(errors.Count == 0, Format(...)), so the message builder runs on a pass. The missing-XML-doc yield break guard is removed, so XDocument.Load throws FileNotFoundException naming the path instead of silently producing zero rows.
ReadmeExampleCompilationTests	91.2%	Same assertion change. LocateReadme is a single while loop and returns a path that doesn't exist when nothing is found, so ReadAllText fails loudly. The old version returned a bare "README.md" and yielded nothing.
PropertyTests	97.7%	The whitespace-message skip was a workaround for #273, which is closed. The property now asserts the type-name fallback instead of returning true, so it tests more than before.
RunAsyncFuncTests	96.1%	The mid-flight cancellation probe had an unreachable return 42; after Task.Delay(Infinite). It now returns a TaskCompletionSource that the token cancels, and still asserts TaskCanceledException. The two already-canceled tests add a positive control: they invoke the probe afterwards and assert it flips, which proves the Assert.False(wasInvoked) isn't vacuous.

Heads-up, test-code changes: these edit existing tests, so please review the table above.

Verification

• dotnet build -c Release -p:TreatWarningsAsErrors=true, test project, all 10 TFMs: 0 errors.
• dotnet test --filter "Category!=Fuzz" with --settings coverlet.runsettings: net10.0 145 passed, net48 106 passed. Merged ReportGenerator summary: Wolfgang.TryPattern 100%, Wolfgang.TryPattern.Tests.Unit 100%, no class under 100%.

Not in this PR (remaining #310 steps)

• Step 4: the Stage 1 gate in pr.yaml still reads with a bare while read -r line and gates everything at CODECOV_MINIMUM (90%). repo-template already has the fix (IFS= plus a 100% test-assembly gate). It's a protected-file change, so it goes in a separate bypass PR.
• Step 5: release.yaml parity.

Refs #310

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