Prefer a complete generic argument mapping for multi-interface open generics (#1464)

[tillig]

Summary

Fixes #1464. When an open-generic implementation implements multiple interfaces matching the registered service pattern, and the first matching interface candidate could not actually map its generic arguments to the implementation's type parameters (e.g. IHandler<IRequest> appearing before IHandler<IRequest<TParam>>), TryMapImplementationGenericArguments returned that broken candidate via the availableArguments[0] fallback. The result was a valid registration being silently dropped from resolved enumerables.

Fix

In OpenGenericServiceBinder.TryMapImplementationGenericArguments, when there is no exact match, the binder now prefers the first candidate that produces a complete mapping (every implementation type parameter resolved — no null entries), falling back to availableArguments[0] only when no candidate maps completely. This preserves prior behavior for genuinely un-bindable cases. The single-interface and exact-match paths are unchanged.

The "complete mapping" check (a.All(arg => arg is not null)) mirrors the check the caller already performs, keeping the semantics consistent.

Second case (IHandler<int> registered as IHandler<>)

A type like Handler<TParam> : IHandler<int> registered against the open IHandler<> still correctly produces no resolution result (the closed int argument leaves TParam un-inferable), with no exception and no incorrect binding — same as before the fix. If registration-time rejection of such cases is desired, that's a separate issue.

Tests

Adds 4 tests in OpenGenericMultipleInterfaceOrderTests: mappable-interface-first resolves; non-mappable-interface-first resolves (the #1464 regression); both order variants resolve from a single enumerable; and the unrelated-type-param case resolves to empty. All 155 existing open-generic tests pass.

No public API changes. Zero warnings/errors; full suites pass on net8.0 and net10.0.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.91%. Comparing base (f8d4095) to head (4597770).
⚠️ Report is 9 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1489      +/-   ##
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+ Coverage    77.69%   77.91%   +0.21%     
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  Files          217      217              
  Lines         5829     5877      +48     
  Branches      1253     1259       +6     
===========================================
+ Hits          4529     4579      +50     
+ Misses         764      760       -4     
- Partials       536      538       +2     

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