chore: bring over passing async-eager tests (#19011)

more tests from #18880 that already pass on `main`. On that branch,
there are four new tests that use `$state.eager`:

1. async-eager-batch-no-merge
2. async-eager-new-in-earlier-batch
3. async-eager-read-new-dependency
4. async-state-eager-not-connected

1 describes undesirable behaviour (see
https://github.com/sveltejs/svelte/pull/18880#discussion_r4271899616).
The other three already pass on `main` — the changes to the
implementation on #18880 (the `is_eager` stuff) are counteracting other
things that changed on that branch.

Of those, this brings over 2 and 4. 3 is unnecessary — the
`$state.eager` in 3 isn't doing anything (`show` doesn't have any async
dependents).

packages/svelte/tests/runtime-runes/samples/async-eager-new-in-earlier-batch/_config.js

@@ -0,0 +1,33 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+// The eager expression is created while the earlier batch is processed, in whose view `x` is
+// still 0. It reads the real (latest) value nonetheless, so it should not be re-run later on
+export default test({
+	async test({ assert, target, logs }) {
+		await tick();
+		const [a, x, shift] = target.querySelectorAll('button');
+
+		a.click();
+		await tick();
+		x.click();
+		await tick();
+		assert.deepEqual(logs, []);
+
+		shift.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			'<button>a</button><button>x</button><button>shift</button><p>1</p><span>0</span>'
+		);
+		assert.deepEqual(logs, [1]);
+
+		shift.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			'<button>a</button><button>x</button><button>shift</button><p>1</p><span>1</span>'
+		);
+		assert.deepEqual(logs, [1]);
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-eager-new-in-earlier-batch/main.svelte

@@ -0,0 +1,26 @@
+<script>
+	let a = $state(0);
+	let x = $state(0);
+
+	const queued = [];
+
+	function delay(v) {
+		if (!v) return v;
+		return new Promise((r) => queued.push(() => r(v)));
+	}
+
+	function log(v) {
+		console.log(v);
+		return v;
+	}
+</script>
+
+<button onclick={() => a++}>a</button>
+<button onclick={() => x++}>x</button>
+<button onclick={() => queued.shift()?.()}>shift</button>
+
+{#if await delay(a)}
+	<p>{$state.eager(log(x))}</p>
+{/if}
+
+<span>{await delay(x)}</span>

packages/svelte/tests/runtime-runes/samples/async-state-eager-not-connected/_config.js

@@ -0,0 +1,34 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+const buttons = `
+	<button>count</button>
+	<button>eager</button>
+	<button>plain</button>
+	<button>shift</button>
+`;
+
+// Reading a value via `$state.eager` sees the latest value, unlike a regular read, which
+// sees the value from before the (pending) batch wrote it until that batch commits
+export default test({
+	async test({ assert, target }) {
+		await tick();
+		const [count, eager, plain, shift] = target.querySelectorAll('button');
+
+		count.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `${buttons}<p>0</p>`);
+
+		eager.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `${buttons}<p>0</p><span>1</span>`);
+
+		plain.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `${buttons}<p>0</p><span>1</span><b>0</b>`);
+
+		shift.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `${buttons}<p>1</p><span>1</span><b>1</b>`);
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-state-eager-not-connected/main.svelte

@@ -0,0 +1,27 @@
+<script>
+	let count = $state(0);
+	let eager = $state(false);
+	let plain = $state(false);
+
+	const deferred = [];
+
+	function delay(value) {
+		if (!value) return value;
+		return new Promise((resolve) => deferred.push(() => resolve(value)));
+	}
+</script>
+
+<button onclick={() => count++}>count</button>
+<button onclick={() => (eager = true)}>eager</button>
+<button onclick={() => (plain = true)}>plain</button>
+<button onclick={() => deferred.shift()?.()}>shift</button>
+
+<p>{await delay(count)}</p>
+
+{#if eager}
+	<span>{$state.eager(count)}</span>
+{/if}
+
+{#if plain}
+	<b>{count}</b>
+{/if}