Add documenting tests for KeyFilter + class interception limitation (#56)

[tillig]

Relates to #56. Filed upstream as castleproject/Core#748.

Background

Issue #56 reports that [KeyFilter] with an enum key is ignored when a type is registered with both WithAttributeFiltering() and EnableClassInterceptors().

After investigation, the root cause is an upstream Castle DynamicProxy bug, not something in this library or in Autofac core:

• EnableClassInterceptors rewrites the implementation type to a Castle proxy subclass, replicating the original constructor parameter attributes onto the proxy constructor.
• KeyFilterAttribute's constructor parameter is typed object (KeyFilterAttribute(object key)), so an enum key is boxed. Castle reproduces that boxed enum argument using the enum's underlying integer type rather than the original enum type.
• The replicated KeyFilterAttribute.Key ends up as boxed (int)0 instead of e.g. LoggerKey.First, so core's WithAttributeFiltering looks up new KeyedService((int)0, typeof(string)), which doesn't match the registered keyed service. Filtering silently misses.

This only affects non-string keys under class interception. String keys round-trip through object losslessly, and interface interception does not rewrite the constructor, so both are unaffected.

Filed upstream with a minimal, Autofac-free repro: castleproject/Core#748 (not fixed in the latest released Castle.Core 5.2.1, nor on their unreleased 6.0.0 branch).

This PR

Rather than ship a fragile re-injection workaround that duplicates core's filtering logic, this adds a documenting test fixture (ClassInterceptorsWithAttributeFilteringFixture) that:

• Pins the current (incorrect) behavior for an enum key with class interception. If Castle fixes the upstream bug, this test will start failing — signaling that the limitation (and its docs/comments) can be removed.
• Establishes the baseline (enum filtering works without interception).
• Demonstrates the two supported workarounds: use a string key, or use EnableInterfaceInterceptors instead of EnableClassInterceptors.

The class-level XML doc captures the root cause and links both #56 and castleproject/Core#748 so the rationale travels with the code.

Validation

• 46/46 tests pass on net8.0 and net10.0.
• Clean Release build (warnings-as-errors + analyzers) and dotnet format clean.

No production code changes — tests and documentation only. Suggest leaving #56 open to track the upstream fix.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.71%. Comparing base (f79fa27) to head (1cfeacd).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop      #65   +/-   ##
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  Coverage    90.71%   90.71%           
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  Files            2        2           
  Lines          140      140           
  Branches        23       23           
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  Hits           127      127           
  Misses           6        6           
  Partials         7        7           

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