Add generator output unit tests for trimmable typemap (attribute-shape verification)

[simonrozsival]

Problem

The CoreCLRTrimmable lane failure investigated in PR #11091 had a single root cause: the trimmable typemap source generator emitted proxy classes that were missing the self-application [JavaPeerProxy] attribute needed for AOT-safe instantiation via proxyType.GetCustomAttribute<JavaPeerProxy>().

This bug shipped because tests/Microsoft.Android.Sdk.TrimmableTypeMap.Tests/ verifies generator inputs and high-level structure but does not verify that each emitted proxy type has the attribute shape runtime lookup depends on. The bug would have been caught instantly by a test like "for every emitted proxy type, Type.GetCustomAttribute<JavaPeerProxy>() returns an instance pointing at that same type."

Proposal

Extend tests/Microsoft.Android.Sdk.TrimmableTypeMap.Tests/ with attribute-shape verification:

1. Self-application attribute test: For every generated JavaPeerProxy-derived type, assert:

   • type.GetCustomAttribute<JavaPeerProxy> (inherit: false) is non-null.
   • The attribute's TargetType equals the type itself (or the expected target).
   • JniName matches the expected JNI name for the bound Java type.

2. Generated-vs-expected attribute matrix: For a curated set of inputs (Java.Lang.Object, Android.Widget.Button, an interface with invoker, a managed-only subclass, a desugar-annotated type), snapshot the expected attribute set on the generated proxy and assert it matches.

3. Compile-and-load verification: Actually emit the generator output, compile the resulting .dll, load it, enumerate JavaPeerProxy-derived types, and run the above assertions on the loaded metadata (not just the source text).

Acceptance criteria

• Tests exist in tests/Microsoft.Android.Sdk.TrimmableTypeMap.Tests/ that would have failed against the generator state that caused PR [TrimmableTypeMap] Add CoreCLRTrimmable Mono.Android.NET-Tests lane #11091's CoreCLRTrimmable failures.
• Tests run as part of the standard dotnet test flow (already wired into CI).
• Coverage includes at least: concrete class, interface with invoker, abstract class, managed-only subclass, Desugar-prefixed type.

Discovered during

PR #11091 — the CoreCLRTrimmable lane was failing because generated proxies lacked self-application [JavaPeerProxy] attributes. The generator source looked correct; the bug was only visible by decompiling the emitted .dll.