fix(subscription): delete bookkeeping Map entries on last unmount

[spokodev]

Fixes #4261.

The cleanup callback at src/subscription/index.ts:84-93 calls dispose() when the last subscriber for a key unmounts, but never removes the matching entries from the subscriptions and disposers Maps. In a long-lived app that cycles through many distinct subscription keys (paginated feeds, rotating live-data channels, user-navigated rooms), both Maps accumulate one dead entry per unique key and grow without bound.

This change deletes the entry from both Maps right after calling dispose():

if (!count) {
  const dispose = disposers.get(subscriptionKey)
  dispose?.()
  subscriptions.delete(subscriptionKey)
  disposers.delete(subscriptionKey)
}

The functional behaviour is unchanged: a subsequent re-mount for the same key still hits the if (!refCount) branch (because subscriptions.get(key) || 0 is 0 whether the entry is missing or zero), so a fresh subscribe runs and a fresh disposer is registered.

Tests

test/use-swr-subscription.test.tsx gets a teardown cleans up bookkeeping Maps (#4261) describe block with 7 new cases covering both axes of the fix.

Because the bookkeeping Maps are module-private (closed over inside subscription/index.ts via a per-cache WeakMap), a minimal @internal helper is exported so the tests can read their sizes:

export const _unstable_getSubscriptionStateSize = (
  cache: object
): { subscriptions: number; disposers: number } | null

The naming follows the standard _unstable_ prefix convention and the helper is annotated @internal so it does not enter the documented public API. Suggestions on a better location welcome.

Positive (the fix's contract)

• After the last subscriber unmounts, both Maps drop the entry (sizes go to 0).

Negative (existing behavior preserved)

• While another subscriber is still mounted, the entry stays in both Maps (refcount > 0).
• dispose() is still called exactly once on the final unmount.
• Re-mounting the same key after teardown runs a fresh subscribe and produces a fresh disposer.

Edge

• Distinct keys are isolated. Unmounting one does not affect another active subscription on the same cache.
• N mount/unmount cycles on distinct keys (the leak scenario from the issue) leave the Maps at size 0.
• Per-cache scope. Cleaning up one SWRConfig's subscription does not touch the bookkeeping of another SWRConfig.

All 7 fail on main against the relevant assertions and pass on this branch. The 7 pre-existing tests in the same file are unchanged and still pass.

Verification

pnpm test
# 367 passed, 5 skipped (372)

pnpm test test/use-swr-subscription.test.tsx
# 14 passed

pnpm types:check
# clean

pnpm exec eslint src/subscription/index.ts test/use-swr-subscription.test.tsx
# clean

pnpm prettier --check src/subscription/index.ts test/use-swr-subscription.test.tsx
# All matched files use Prettier code style!

Opened as draft until CI is green.