fix(subscription): support async subscribe function in useSWRSubscription

[wasim-builds]

Description

Closes #4214

Problem

Previously, useSWRSubscription executed subscribe(args, { next }) synchronously and asserted that the returned disposer was a function via if (typeof dispose !== 'function') throw new Error(...). When subscribe is an async function, it returns a Promise instead of an unsubscribe function, causing the hook to immediately throw an error. In addition, any cleanup function returned upon promise resolution was never registered or called, causing connection and memory leaks.

Root Cause

1. SWRSubscription type in src/subscription/types.ts did not explicitly type Promise<(() => void) | void> as an allowed return type.
2. In src/subscription/index.ts, subscribe was assumed to return a synchronous cleanup function. Promise returns were treated as invalid return values, and asynchronous resolution and unmount cleanup lifecycles were not handled.

Fix

1. Updated SWRSubscription in src/subscription/types.ts to allow (() => void) | void | Promise<(() => void) | void> as return types.
2. In src/subscription/index.ts:
   • Supported Promise returns (dispose && typeof dispose.then === 'function').
   • Tracked unmount state with a disposer flag. If the component or last subscriber unmounts before the async subscribe promise resolves, the returned cleanup function is immediately invoked upon settlement to prevent connection leaks.
   • If still mounted, the resolved cleanup function is registered in disposers.
   • Handled promise rejections by forwarding errors to next(error) if the subscription remains active.
   • Preserved synchronous cleanup functions and allowed void returns.
3. Added unit tests in test/use-swr-subscription.test.tsx verifying:
   • Async subscribe function with cleanup function called on unmount.
   • Async subscribe function where unmount occurs before promise resolves (immediate cleanup on resolution).
   • Async subscribe function returning void without throwing on unmount.
   • Async subscribe function rejection forwarding error to next.
   • Deduplication across multiple subscribers with async subscribe functions.

Verification

• pnpm jest test/use-swr-subscription.test.tsx: 1 passed, 13 passed, 13 total.
• pnpm test-typing: Clean pass.
• pnpm test: 36 passed, 402 passed, 5 skipped, 407 total.

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