Add partial state mode support to the RPC API. In partial state mode:
- Account queries (balance, nonce, account proofs) work for ALL accounts
- Storage/code queries only work for tracked contracts
- Clear error codes help clients understand limitations
Changes:
- New error types: StorageNotTrackedError (-32001), CodeNotTrackedError (-32002)
- Backend interface: PartialStateEnabled(), IsContractTracked()
- Modified RPCs: GetStorageAt, GetCode, GetProof check tracked status
- 7 new tests verify correct behavior for tracked/untracked contracts