rpc: implement pending resolver and GetPendingTransactions GraphQL

[lupin012]

The GraphQL server had three pending field resolvers and the query.pending field itself all implemented with panic("not implemented") — any GraphQL call on pending would crash.

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Changes

rpc/jsonrpc/graphql_api.go

• Added txPool txpoolproto.TxpoolClient as a dependency of GraphQLAPIImpl
• Added GetPendingTransactions(ctx) to the GraphQLAPI interface and its implementation: calls txPool.Pending() via gRPC, decodes RLP-encoded transactions, and returns them as []types.Transaction
• Nil-safe: if txPool is nil, returns an empty list without error

rpc/jsonrpc/daemon.go

• Updated the NewGraphQLAPI(...) call to pass txPool as the new parameter

cmd/rpcdaemon/graphql/graph/schema.resolvers.go

• queryResolver.Pending: fetches pending txs and populates model.Pending with TransactionCount and the Transactions list (nonce, gas, hash)
• pendingResolver.Account: resolves the account at pending state via GetAccountInfo(..., PendingBlockNumber)
• pendingResolver.Call: executes an eth call at pending state via PendingBlockNumber
• pendingResolver.EstimateGas: estimates gas at pending state via PendingBlockNumber

cmd/rpcdaemon/graphql/graph/schema_resolvers_test.go

• Added GetPendingTransactions to the mockGraphQLAPI mock
• 6 new tests: empty pool, one pending tx, error propagation, invalid address, PendingBlockNumber forwarding for Call and EstimateGas