[wolf4ood]

What this PR changes

• Control Plane > Reliable Messaging: how a connector answers protocol messages (400 for an invalid state transition, 404 when the process does not exist or the client is not authorized, 401 for catalog requests, 503 when the message can't be processed now), and how the sender reacts to 4xx, 5xx and timeouts.
• Data Plane > Data Plane Signaling: which failed signals are retried by the state machine, and why slow data planes should answer asynchronously.
• State Machines Known Limitations: a signal whose response was lost is rejected by the data plane when sent again, and retries select the data plane again.
• Contributors > Data plane integration: how signaling failures map to ERROR_RETRY / FATAL_ERROR; the provider sends start from INITIAL, not STARTING.

Why

Further notes

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signature changes, package declarations, bugs that were encountered and were fixed inline, etc.

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Linked Issue(s)

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