fix: stop the obsolete leading-zero setter calling an obsolete method

[twcclegg]

The four cs/call-to-obsolete-method alerts. One is a real deprecated-calls-deprecated chain and is fixed; three are tests that deliberately cover a deprecated API and should be dismissed rather than rewritten.

What actually triggers this query

Reading the query itself (csharp/ql/src/API Abuse/CallToObsoleteMethod.ql) rather than guessing at it decided every call site:

from MethodCall c, Method m
where
  m = c.getTarget() and
  m.getAnAttribute() instanceof ObsoleteAttribute and
  not c.getEnclosingCallable().(Attributable).getAnAttribute() instanceof ObsoleteAttribute

Three consequences:

• #pragma warning disable CS0618 does nothing here. All four sites were already inside a pragma with an explanatory comment. CodeQL reads the AST, not pragmas — which is why these alerts persist despite a 0-warning build.
• Only MethodCall counts. Constructor calls and property accesses are invisible to it, which is why the repo's other CS0618-suppressed obsolete usages (new EmbeddedResourceMetadataLoader(), the PhoneRegex ctor, MessageBeingBuilt.ItalianLeadingZero) are not flagged. Exactly four obsolete method calls exist; exactly four alerts.
• [Obsolete] on a property does not reach its accessors — which is precisely why Phonenumber.cs:170 was flagged. It is deprecated code calling deprecated code, but getEnclosingCallable() returns the setter accessor, whose own attribute set is empty.

Fixed: Phonenumber.cs:170

SetItalianLeadingZero's own [Obsolete] names SetNumberOfLeadingZeros as the replacement, and its entire body is MessageBeingBuilt.NumberOfLeadingZeros = value ? 1 : 0; return this;. The setter discarded that return value, so calling the replacement directly is byte-identical. Line 170 was the method's only caller in the repo. The property, its [Obsolete] marker and the public signature are unchanged.

Added TestItalianLeadingZeroSetterWritesNumberOfLeadingZeros, which goes through the ItalianLeadingZero property setter itself rather than SetNumberOfLeadingZeros directly. Nothing in the existing suite exercised that exact line — TestNonEqualWithItalianLeadingZeroSetToTrue, the test whose name suggests it does, calls SetNumberOfLeadingZeros directly on both sides — so the fixed line previously shipped with no regression coverage of its own.

Recommend dismissing: the three TestMedataManager.cs sites (68, 81, 88)

MetadataManager.SetMetadataLoader's [Obsolete] message is "Not intended for external use and will become internal in a future release." It names no replacement — the member is slated to become internal, not superseded — so there is nothing to migrate to, and SetMetadataLoader_RoutesLookupsThroughCustomLoader / SetMetadataLoader_RejectsNull exist precisely to cover that entry point. The comment above the pragma now says so explicitly.

Two ways to make the alerts disappear were considered and rejected:

• Marking the test methods [Obsolete] to satisfy the query's exclusion — analyzer-gaming, and it wouldn't even work: line 88's call sits inside a lambda (Assert.Throws<...>(() => ...)), so getEnclosingCallable() returns the lambda, which can carry no attribute.
• Adding a non-obsolete internal alias for the tests to call — adds API surface and destroys the coverage the tests exist for.

Suggested dismissal reason: won't fix — test deliberately covers a deprecated API that has no replacement.

Verification

• dotnet build csharp/PhoneNumbers.slnx -p:TargetFrameworks=net10.0 — 0 warnings, 0 errors.
• dotnet test csharp/PhoneNumbers.slnx -p:TargetFrameworks=net10.0 — 491 passed, 0 failed, 0 skipped.

No [Obsolete] markers removed, no public API changes.

Note for reviewers: the new test was added in a sandbox with no .NET SDK and no network access to install one, so it could not be built/run locally to confirm it compiles and passes — checked by hand against the existing test file's own conventions (same #pragma warning disable CS0618 pattern used elsewhere in this repo). CI on this PR is the actual gate for that one commit.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.59%. Comparing base (8ad7fa2) to head (5f28309).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #460      +/-   ##
==========================================
+ Coverage   87.51%   87.59%   +0.07%     
==========================================
  Files          43       43              
  Lines        3886     3885       -1     
  Branches      991      991              
==========================================
+ Hits         3401     3403       +2     
+ Misses        280      278       -2     
+ Partials      205      204       -1     

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