feat: wire keyring_createAccounts (Batch accounts 4/5)

[gantunesr]

Stack part 4 of 5 split from #601. Routes keyring_createAccounts, validates supported BIP-44 index/index-range options, chunks ranges internally in batches of 100, and maps responses back to KeyringAccount objects.

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Note

Medium Risk
Introduces a new RPC pathway for batch account creation and adds input validation plus internal chunking logic, which could affect account provisioning behavior and ordering/trace handling if edge cases are missed.

Overview
Adds support for the keyring_createAccounts RPC in KeyringHandler, validating that only Bip44DeriveIndex and Bip44DeriveIndexRange are accepted and enforcing non-negative, inclusive index ranges.

Implements batched account creation via AccountUseCases.createMany, hard-coding v1 constraints (bitcoin mainnet + P2WPKH only), splitting requests into internal chunks of up to 100, and wrapping the operation in snap tracing (Create Bitcoin Accounts Batch) with endTrace guaranteed in finally.

Extends KeyringHandler.test.ts with comprehensive coverage for range handling, ordering, batching behavior, unsupported types, address-type constraints, error propagation, and tracing; adds a small expectDefined helper for type narrowing in tests.

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