fix: restore isCurrentlyOwned when re-adding an NFT

[SnowingFox]

Description

Fixes #9787

When an NFT leaves a wallet and is later re-acquired, the manual import path
reports success but the NFT stays invisible. #addMultipleNfts refreshes an
existing state entry with { ...existingEntry, ...nftMetadata }, and because
nftMetadata never carries isCurrentlyOwned, the stale isCurrentlyOwned: false written earlier by checkAndUpdateSingleNftOwnershipStatus survives the
merge.

Ownership is already proven on the manual import path: addNftVerifyOwnership
throws unless the user owns the token on-chain, and addNft/addNfts default
to Source.Custom when invoked for manual imports. This PR restores
isCurrentlyOwned: true when refreshing an existing entry on the
Source.Custom path, so re-acquired NFTs reappear. Other sources (autodetection
Source.Detected, dapp-suggested Source.Dapp) are untouched.

Changes

• packages/assets-controllers/src/NftController.ts: restore isCurrentlyOwned
  to true in #addMultipleNfts when source === Source.Custom.
• packages/assets-controllers/src/NftController.test.ts: add a regression test
  that re-adds a previously-unowned NFT via addNftVerifyOwnership and asserts
  isCurrentlyOwned flips back to true.
• packages/assets-controllers/CHANGELOG.md: add [Unreleased] Fixed entry.

Test plan

cd packages/assets-controllers
NODE_OPTIONS=--experimental-vm-modules yarn jest src/NftController.test.ts src/NftDetectionController.test.ts --coverage=false

• New regression test fails on main (Expected: true, Received: false) and
  passes with the fix.
• NftController.test.ts: 117 passed, 4 snapshots.
• NftDetectionController.test.ts: 20 passed.
• eslint clean on changed files; changelog:validate passes.

---

Note

Low Risk
Narrow fix to manual NFT import merge logic with a targeted regression test; no auth, security, or broad behavioral changes outside Custom source updates.

Overview
Fixes re-acquired NFTs staying hidden after a successful manual import when state still had isCurrentlyOwned: false from an earlier ownership check.

In #addMultipleNfts, updating an existing entry now sets isCurrentlyOwned: true when source === Source.Custom (manual import / addNftVerifyOwnership), instead of leaving the stale flag from the spread merge. Autodetection and dapp-suggested sources are unchanged.

Adds a regression test for addNftVerifyOwnership and an [Unreleased] changelog entry.

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