[Java.Interop] Defer exception creation in TryLoadClassWithFallback to fix global ref leak

[simonrozsival]

Summary

Fixes the JNI global reference leak introduced by #1407 by deferring managed exception creation in TryLoadClassWithFallback.

Root Cause

PR #1407 added a Class.forName() fallback to the UTF-8 FindClass path, sharing TryLoadClassWithFallback between the string and ReadOnlySpan<byte> overloads.

The problem: TryLoadClassWithFallback eagerly calls GetExceptionForThrowable on the FindClass failure throwable before trying Class.forName(). This creates a full JavaException managed object which:

1. Promotes the JNI local ref to a global ref (via ConstructPeer)
2. Calls getMessage(), getCause() (recursively creating more JavaExceptions with their own global refs), and getStackTrace()
3. All of this happens even when Class.forName() succeeds immediately after, or when throwOnError=false

Before #1407, the UTF-8 path never entered this code, so no managed exceptions were created for failed FindClass calls. After #1407, every UTF-8 class lookup failure eagerly creates (and disposes) managed exceptions with global refs — causing the global ref churn and OOM observed in Debug mode on Android.

Fix

Defer managed exception creation: try Class.forName() first using only raw JNI operations, and only materialize the managed exception if we actually need to throw.

Code path	Before (old)	After (new)
Class.forName() succeeds	Create JavaException + global ref, then Dispose()	Delete local ref only — no managed exception, no global ref
throwOnError=false (both fail)	Create JavaException + global ref, then Dispose()	Delete local ref only — no managed exception, no global ref
throwOnError=true (both fail)	Create JavaException + throw	Same — create JavaException + throw

Testing

• All 667 Java.Interop-Tests pass, including all 16 UTF-8 FindClass tests added in Fix UTF-8 JniType class lookup fallback #1407.
• Added two new regression tests (TryFindClass_Utf8_DoesNotLeakGlobalRefs, TryFindClass_String_DoesNotLeakGlobalRefs) that assert GlobalReferenceCount is unchanged after a TryFindClass call for a non-existent class.

[Copilot]

Pull request overview

This PR updates JniEnvironment.Types class lookup logic to avoid creating managed JavaException instances (and thus JNI global refs) unless an exception must actually be thrown, addressing a global reference leak/churn scenario in the UTF-8 FindClass fallback path.

Changes:

• Defer managed exception materialization in TryLoadClassWithFallback until throwOnError=true and all fallback attempts fail.
• Convert dot-separated class names to JNI slash form before calling raw FindClass (string and UTF-8 overload paths).
• Refactor UTF-8 TryFindClass to share a pointer-based helper and perform dot-to-slash conversion with stack/heap buffer as needed.