Add GC.KeepAlive to ComposeBridges JNI calls

[jonathanpeppers]

The raw-JNI bridges in ComposeBridges.cs were missing the GC.KeepAlive pattern that dotnet/java-interop emits for every bound member. Once ((Java.Lang.Object)obj).Handle is read into a JValue, the JIT considers the managed wrapper dead, so a GC during the JNI call can finalize it and invalidate the underlying handle. In practice it usually doesn't crash because the lambda wrappers are referenced from ComposableLambda*/the call stack and Compose calls are short, but under GC pressure it's exactly the kind of bug that surfaces as a "stale local reference" JNI error.

What changed

Every bridge (Text, Button, IconButton, FloatingActionButton, Surface, TextField/OutlinedTextField, AlertDialog, Card, AssistChip/FilterChip/InputChip/SuggestionChip, NavigationBar/NavigationBarItem, NavigationRail/NavigationRailItem) now wraps JNIEnv.CallStaticVoidMethod in a try { ... } finally { GC.KeepAlive(...); } block, with one KeepAlive per managed parameter whose .Handle was read into a JValue. The bridges that already had a try/finally to DeleteLocalRef a JNIEnv.NewString ref (Text, InvokeTextField) fold the new KeepAlive calls into that existing finally rather than nesting a second one.

GC.KeepAlive(null) is a no-op so the optional slot wrappers in AlertDialog, the chips, and the nav items are kept alive unconditionally without a null check.

Instructions update

Added rule #7 under the JNI-bridges section of .github/copilot-instructions.md documenting the try/finally + GC.KeepAlive shape, the rationale (matches dotnet/java-interop output), and a copy-paste pattern, so future bridges get it right from the first commit.

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com

[Copilot]

Pull request overview

This PR hardens the raw JNI bridge layer (ComposeBridges.cs) by ensuring managed wrapper objects (lambdas and the IComposer) remain alive across JNIEnv.CallStaticVoidMethod invocations, matching the lifetime-safety pattern emitted by dotnet/java-interop. This reduces the risk of stale/invalid JNI handles under GC pressure.

Changes:

• Wrapped each JNIEnv.CallStaticVoidMethod in try/finally and added GC.KeepAlive(...) for every managed parameter whose .Handle is captured into a JValue.
• Folded the GC.KeepAlive(...) calls into existing try/finally blocks where string DeleteLocalRef cleanup already existed (e.g., Text, InvokeTextField).
• Documented the required try/finally + GC.KeepAlive JNI-bridge pattern in .github/copilot-instructions.md to prevent future regressions.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/ComposeNet.Compose/ComposeBridges.cs	Adds try/finally wrappers and GC.KeepAlive(...) calls around JNI invocations to prevent premature finalization of managed wrappers during JNI calls.
.github/copilot-instructions.md	Documents the required JNI-bridge lifetime-safety pattern (matching dotnet/java-interop) for future bridge additions.