fix(signals): await non-Promise thenables yielded from action (closes #2765)

[tsushanth]

Closes #2765.

`action.ts` was checking yielded values with `instanceof Promise`. That skips anything whose `then` is a method but whose prototype isn't `Promise.prototype` — userland thenables, cache library handles (the `syncThenable` shape used elsewhere in this same package), and engines that pass cross-realm promises across worker boundaries. Every such yield landed in the synchronous `run(r)` branch, so the generator resumed immediately with the raw thenable object — `got: [object Object]` in the StackBlitz repro.

The fix replaces both `instanceof Promise` checks with a small `isThenable` predicate (`value != null && (object|function) && typeof value.then === "function"`), the same shape `await` itself uses, and casts the awaited path to `PromiseLike<...>`. Native Promises still satisfy the predicate, so the existing test cases for `yield Promise.resolve(42)` etc. keep their behavior — the change only widens what counts as awaitable.

Test

Added `should await a non-Promise thenable and resume with its fulfilled value (#2765)` in `packages/solid-signals/tests/action.test.ts`:

• yields `{ then(onFulfilled) { queueMicrotask(() => onFulfilled(42)); } }`
• asserts the generator does NOT resume synchronously with the raw object (pre-fix bug)
• asserts the resumed value is `42` after the microtask drains

Without the patch the new test fails at `expect(receivedValue).toBe(42)` (receives the thenable object); with the patch it passes. The full `pnpm --filter @solidjs/signals test tests/action.test.ts` suite (57 tests) stays green.

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[ryansolid]

Good catch — action() only awaited native Promise instances, so a yielded thenable that wasn't instanceof Promise resumed the generator with the raw object instead of its settled value. We've landed an equivalent fix on next: yield handling now uses a thenability check (typeof value.then === "function") at both sites, matching how await treats thenables, with a regression test.

The map.ts/repeat change bundled here is already on next (the _offset reset from the earlier repeat fix), so there's nothing to take there. Closing since the action fix is in — thanks for the thenable fix.