fix(runtime): plugin proxy must not be unintentionally Thenable

[playopsstudio]

fix(runtime): plugin proxy must not be unintentionally Thenable

Summary

Adds case 'then', case 'catch', and case 'finally' to the plugin-proxy get-trap switch in core/src/runtime.ts, returning undefined. Aligns with the existing $$typeof short-circuit (which solves the equivalent React-framework-machinery problem; see comment at line 167).

Fixes #8472.

Mechanism (short form; full detail in linked issue)

JS engines treat any object with a callable .then as a Thenable. The proxy's get-trap returns a method-wrapper function for any property name not in the switch — including .then. So Promise.resolve(proxy) invokes proxy.then(resolve, reject), which dispatches a bogus "then" method to the native bridge → either "method not implemented" rejection OR indefinite hang (wrapper never invokes resolve/reject).

This bites code that holds a plugin proxy as a Promise resolution value — most commonly via the lazy-load pattern let p = import("some-plugin").then((m) => m.Plugin), which is a standard dynamic-import idiom for code-splitting Capacitor plugins on tree-shake-aware web targets.

Real-world impact

Three independent production failure surfaces in a private game-runtime codebase, all caused by this single bug — one developer-week of investigation before the root cause was identified. We shipped a consumer-side container-wrap workaround (Promise<{ plugin: PluginType }> instead of Promise<PluginType>), but the workaround is fragile (one missed wrap reintroduces the bug class).

Fix

3-line addition in core/src/runtime.ts; 1 regression test in core/src/tests/plugin.spec.ts.

Test plan

• Existing plugin.spec.ts test suite unchanged behavior (pnpm test in core/)
• New regression test passes: typeof proxy.then === 'undefined' + Promise.resolve(proxy) === proxy
• Verified on web platform via core/'s Jest tests
• Verified on iOS + Android via Capacitor sample app (no behavior change expected; proxy method-dispatch flow unchanged)

Affected versions

Bug verified in @capacitor/core@8.3.1, @capacitor/core@9.0.0-alpha.2, and current main. The proxy structure has been stable across these versions; the fix applies cleanly.

Backward compatibility

Zero behavior change for any code that uses the proxy for its intended purpose (await proxy.someMethod(args)). Eliminates a latent footgun for code that accidentally feeds the proxy through Promise resolution.

Related context

// https://github.com/facebook/react/issues/20030 comment at core/src/runtime.ts:167 already documents the precedent: framework-internal property access (React's $$typeof) needs explicit short-circuit in the proxy. Promise machinery's .then/.catch/.finally checks are architecturally equivalent — same problem class, same solution shape.