Forward unmatched tool.error envelopes to ARCPClient.unhandled instead of dropping them

[nficano]

ARCPClient.dispatch(envelope:) only delivers tool.error envelopes when their correlation_id matches an entry in pendingByInvoke, at Sources/ARCP/Client/ARCPClient.swift:220. If the correlation id is missing or does not map to a pending invocation — for example a runtime error response correlated to a raw client.send invocation that bypassed invoke, an error delivered out of order after the invocation has already been resolved by a job terminal event, or an envelope whose correlation field was stripped by a relay — the dispatcher silently drops it. This is the same shape as the gap addressed for job.progress in issue #55 and leaves real error signals invisible to applications that drain client.unhandled, complicating diagnostics and hiding protocol violations from operators.

Fix prompt: Add an else branch to the .toolError case that yields the envelope to unhandledContinuation whenever there is no matching pending invocation, mirroring how jobCompleted, jobFailed, jobCancelled, and jobAccepted already fall back to unhandled. Add a dispatcher test that sends a tool.error envelope with no correlation_id and asserts it surfaces on client.unhandled, plus a regression test proving that a correlated tool.error still resolves the pending invocation with .failed and does not duplicate onto unhandled.