AVRO-4357: [C#] Resolve schema types without enumerating all loaded types

[gscher]

What is the purpose of the change

This pull request makes resolving the generated class of a named schema in ObjectCreator.FindType independent of the number of types loaded in the process, fixing AVRO-4357. Before, every name not found in the entry assembly enumerated all types of all loaded assemblies, which takes seconds and gigabytes of allocations in large applications on the first deserialization of a schema.

• Names with a namespace are looked up directly with Assembly.GetType in every loaded assembly.
• Names without a namespace, and names not found directly, fall back to the existing enumeration.
• UnMangle(name) is computed once per name instead of once per type.
• The resolved type is the same as before: if several assemblies contain the type, the one found last in the loaded assemblies wins.
• One difference: an assembly whose GetTypes() throws was skipped entirely before. The direct lookup can still find a loadable type in it, so names that failed to resolve before can resolve now.

This touches the same method as #3966 (AVRO-3893), which avoids the per-call closure in FindType. The two changes are independent and should combine cleanly; I'm happy to rebase onto it if it lands first.

Verifying this change

This change added tests and can be verified as follows:

• ObjectCreatorTests gains tests for resolving a full name from a non-entry assembly, for reserved words in the namespace (mangled and unmangled), and for the last loaded assembly winning when several assemblies contain the same full name. All new tests pass before and after the change.
• Avro.benchmark gains ObjectCreatorBenchmarks, which resolves 50 named types with a fresh ObjectCreator while 0 to 250,000 additional types are loaded. The benchmark can be removed if it shouldn't land in main.

Run the benchmark with dotnet run -c Release -f net10.0 -- --filter '*ObjectCreatorBenchmarks*' in lang/csharp/src/apache/benchmark, once with ObjectCreator.cs from main and once with this change.

Before (ObjectCreator.cs from main):

BenchmarkDotNet v0.15.8, macOS Tahoe 26.6.2 (25G83) [Darwin 25.6.0]
Apple M2 Max, 1 CPU, 12 logical and 12 physical cores
.NET SDK 10.0.401
  [Host]     : .NET 10.0.12 (10.0.12, 10.0.1226.42308), Arm64 RyuJIT armv8.0-a
  Job-HDDYAJ : .NET 10.0.12 (10.0.12, 10.0.1226.42308), Arm64 RyuJIT armv8.0-a

IterationCount=10  LaunchCount=1  WarmupCount=2

| Method                  | AdditionalLoadedTypes | Mean       | Error    | StdDev   | Gen0        | Gen1       | Allocated  |
|------------------------ |---------------------- |-----------:|---------:|---------:|------------:|-----------:|-----------:|
| ResolveNamedSchemaTypes | 0                     |   286.9 ms | 14.46 ms |  8.60 ms |  36500.0000 |  1000.0000 |  292.12 MB |
| ResolveNamedSchemaTypes | 25000                 | 1,017.6 ms | 79.68 ms | 52.70 ms | 117000.0000 |  3000.0000 |  939.97 MB |
| ResolveNamedSchemaTypes | 100000                | 2,720.9 ms | 97.21 ms | 57.85 ms | 339000.0000 | 10000.0000 | 2707.81 MB |
| ResolveNamedSchemaTypes | 250000                | 5,970.5 ms | 33.21 ms | 21.96 ms | 782000.0000 | 23000.0000 | 6243.94 MB |

Outliers
  ObjectCreatorBenchmarks.ResolveNamedSchemaTypes: IterationCount=10, LaunchCount=1, WarmupCount=2 -> 1 outlier  was  removed (311.73 ms)
  ObjectCreatorBenchmarks.ResolveNamedSchemaTypes: IterationCount=10, LaunchCount=1, WarmupCount=2 -> 1 outlier  was  removed (3.02 s)

After (this change):

BenchmarkDotNet v0.15.8, macOS Tahoe 26.6.2 (25G83) [Darwin 25.6.0]
Apple M2 Max, 1 CPU, 12 logical and 12 physical cores
.NET SDK 10.0.401
  [Host]     : .NET 10.0.12 (10.0.12, 10.0.1226.42308), Arm64 RyuJIT armv8.0-a
  Job-HDDYAJ : .NET 10.0.12 (10.0.12, 10.0.1226.42308), Arm64 RyuJIT armv8.0-a

IterationCount=10  LaunchCount=1  WarmupCount=2

| Method                  | AdditionalLoadedTypes | Mean       | Error     | StdDev    | Gen0     | Gen1    | Allocated  |
|------------------------ |---------------------- |-----------:|----------:|----------:|---------:|--------:|-----------:|
| ResolveNamedSchemaTypes | 0                     |   592.1 us |  12.87 us |   7.66 us |  78.1250 |  1.9531 |  641.41 KB |
| ResolveNamedSchemaTypes | 25000                 |   986.0 us |  25.57 us |  15.22 us | 126.9531 |  1.9531 |  1041.8 KB |
| ResolveNamedSchemaTypes | 100000                | 2,156.7 us |  35.80 us |  23.68 us | 273.4375 |  7.8125 | 2242.98 KB |
| ResolveNamedSchemaTypes | 250000                | 4,446.9 us | 293.64 us | 194.23 us | 562.5000 | 15.6250 | 4645.32 KB |

Outliers
  ObjectCreatorBenchmarks.ResolveNamedSchemaTypes: IterationCount=10, LaunchCount=1, WarmupCount=2 -> 1 outlier  was  removed (651.12 us)
  ObjectCreatorBenchmarks.ResolveNamedSchemaTypes: IterationCount=10, LaunchCount=1, WarmupCount=2 -> 1 outlier  was  removed (1.05 ms)

Documentation

• Does this pull request introduce a new feature? No