test(spend-permission): add coverage for fetchPermission

[anilkaracay]

Problem

fetchPermission.ts (spend-permission utility) had no co-located test coverage,
unlike its sibling fetchPermissions.ts (plural), which has full tests.

Change

Adds fetchPermission.test.ts with characterization tests for the existing behavior.
No production code is changed. Tests mirror the conventions of the existing
fetchPermissions.node.test.ts (same mocking of :util/provider.js, provider stub,
assertion style) and import the wrapped export.

Coverage:

• with provider: correct coinbase_fetchPermission request and unwrapped .permission result; error propagation
• without provider: request via fetchRPCRequest to CB_WALLET_RPC_URL; error propagation
• return shape: unwraps .permission; handles a minimal permission
• current behavior on malformed RPC responses (characterization, explicitly labelled as
  documenting today's behavior, not asserting it is correct): null response rejects;
  response without a permission key resolves undefined

Testing

• Added 10 tests in .../spend-permission/methods/fetchPermission.test.ts
• yarn test --run: full suite green (account package 74 files / 1010 tests)
• yarn format: clean (Biome)
• yarn build:packages: type-checks

Notes

Tests only. The malformed-response cases are characterization tests documenting current
behavior for future reference; they do not change any behavior.

[cb-heimdall]

🟡 Heimdall Review Status

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