test(client): full-stack integration tests (client + builder + SdkWallet) (#920)

Wire the real client + PrivacyBuilder + SdkWallet with only the prover and paymaster faked (the true
externals). Covers: the private flow (a withdraw → apply_action with the fee withdraw appended and
proved in order, one broadcast); a deposit auto-switching to invoke_and_apply_action with a
user-signed approve; open-note + invoke with placeholders flowing through unresolved; and simulate
rejecting on the SDK path (fee estimation deferred). An open note is only exercised paired with the
invoke that fills it (a lone open note carries no amount). 4 tests.

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