chore: make token fetching more robust

[MoMannn]

Description

What is the reason for the change?

Token metadata fetching can fail when the Account API returns responses with missing optional fields or experiences transient network errors, causing permission requests to fail unnecessarily.

What is the improvement/solution?

1. Relaxed Zod schema validation: Made the TokenBalanceItemSchema more robust by only requiring fields actually used in the code (address, balance, decimals, symbol, name) while marking API metadata fields as optional (object, type, occurrences, chainId, timestamp).

2. Automatic client fallback: Implemented a fallback mechanism in TokenMetadataService that automatically tries the blockchain client when the Account API fails with network or service errors. The fallback is also done in a way that can easily add additional fallback options.

Manual testing steps

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Screenshots/Recordings

Before

After

Pre-merge author checklist

• I've followed MetaMask 7715 Permissions Snaps Contributor Docs and MetaMask 7715 Permissions Snaps Coding Standards.
• I've completed the PR template to the best of my ability
• I’ve included tests if applicable
• I’ve documented my code using JSDoc format if applicable

Pre-merge reviewer checklist

• I've manually tested the PR (e.g. pull and build branch, run the app, test code being changed).
• I confirm that this PR addresses all acceptance criteria described in the ticket it closes and includes the necessary testing evidence such as recordings and or screenshots.

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Note

Relax token balance schema and add client fallback in TokenMetadataService to make token fetching more resilient.

• Services:
  • TokenMetadataService:
    • #getTokenMetadataClientForChainId now returns an ordered list of clients (Account API first if supported, then blockchain client).
    • getTokenBalanceAndMetadata iterates clients with automatic fallback and throws the last error if all fail; logs failures; uses InternalError default.
• Clients:
  • accountApiClient.ts:
    • Relax TokenBalanceItemSchema to require only address, balance, decimals, symbol, name; mark object, type, occurrences, chainId, timestamp as optional.
• Tests:
  • Expand tokenMetadataService.test.ts to cover fallback behavior, unsupported chains, propagation of errors when no fallback, and last-error throwing; maintain existing success and parameter-pass-through cases.

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[jeffsmale90]

Looks good! Minor nit.