wallet: Make fetch injectable in NetworkController initialization #8793

Problem

getRpcServiceOptions in packages/wallet/src/initialization/instances/network-controller.ts hardcodes fetch to globalThis.fetch. Platforms that provide their own fetch implementation — React Native being the primary example — cannot supply it, making the wallet non-portable across those environments.

Proposed direction

Add an optional fetch?: typeof globalThis.fetch field to WalletOptions. The NetworkController init should use options.fetch ?? globalThis.fetch.bind(globalThis) when building getRpcServiceOptions, so the default behaviour is unchanged and platform-specific implementations can be injected at construction time.

Acceptance

• WalletOptions has an optional fetch field.
• getRpcServiceOptions uses the injected fetch when provided, falling back to globalThis.fetch.
• Existing tests pass; a test covers the injected-fetch path.
• Changelog entry added.