[amoeba]

When no entrypoint is specified, the driver manager tries the entrypoint derived from the driver's file name (e.g. AdbcDriverSqliteInit) and then falls back to AdbcDriverInit. On the netstandard2.0 build (used on .NET Framework), the fallback was never tried: that build uses its own NativeLibrary shim, whose GetExport returns IntPtr.Zero for a missing export instead of throwing EntryPointNotFoundException, and the fallback only ran when an exception was thrown. So on .NET Framework, a driver that exports only AdbcDriverInit failed to load with "Unable to find ... export", while the same driver loaded fine on .NET 8 and later.

This PR adds a test for this case and tries the fallback entrypoint whether the first lookup throws or returns IntPtr.Zero.

Bug found and PR created with Opus 5.5.

[amoeba]

Windows CI should fail for 0d42b93.

[amoeba]

Windows CI failed as expected in https://github.com/apache/arrow-adbc/actions/runs/40782252801/job/123146586705?pr=4977:

[xUnit.net 00:00:11.95]     Apache.Arrow.Adbc.Tests.CAdbcDriverImporterFallbackTests.LoadWithFallback_MissingEntrypoint_UsesFallback [FAIL]
  Failed Apache.Arrow.Adbc.Tests.CAdbcDriverImporterFallbackTests.LoadWithFallback_MissingEntrypoint_UsesFallback [20 ms]
  Error Message:
   System.ArgumentException : Unable to find AdbcTestNoSuchEntrypoint export
Parameter name: file
  Stack Trace:
     at Apache.Arrow.Adbc.C.CAdbcDriverImporter.Load(String file, Boolean canUnload, String entryPoint, String fallbackEntryPoint) in D:\a\arrow-adbc\arrow-adbc\csharp\src\Apache.Arrow.Adbc\C\CAdbcDriverImporter.cs:line 103
   at Apache.Arrow.Adbc.C.CAdbcDriverImporter.LoadWithFallback(String file, String entryPoint, String fallbackEntryPoint) in D:\a\arrow-adbc\arrow-adbc\csharp\src\Apache.Arrow.Adbc\C\CAdbcDriverImporter.cs:line 61
   at Apache.Arrow.Adbc.Tests.CAdbcDriverImporterFallbackTests.LoadWithFallback_MissingEntrypoint_UsesFallback() in D:\a\arrow-adbc\arrow-adbc\csharp\test\Apache.Arrow.Adbc.Tests\CAdbcDriverImporterFallbackTests.cs:line 38

[CurtHagenlocher]

Well, that was an embarrassing lack of test coverage on my part :/. Thanks!