fix(subscription): repair missing access-key address indexes

[EfeDurmaz16]

What

• Repair a missing subscription access-key address index whenever an existing primary access-key record is reused.
• Preserve any existing address index.
• Add regression coverage and a patch changeset.

Why

getOrCreateAccessKey returned early for an existing primary record. If the subsequent address-index write failed or the index was lost, retries could never heal it, so getAccessKeyByAddress kept returning null.

How

Both existing and newly-created records now pass through one idempotent ensureAccessKeyAddressIndex helper. It only writes when the index is missing.

Verification

• Subscription store tests: 21/21
• pnpm check:ci
• pnpm check:types

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/mppx@673

commit: 311f490

[brendanjryan]

LGTM -- thanks!