apollo_propeller: handler fixes

[sirandreww-starkware]

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[sirandreww-starkware]

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• apollo_propeller: handler fixes #13291 👈 (View in Graphite)
• apollo_propeller: handler tests #13290
• apollo_propeller: rate-limit invalid unit warnings in handler #13583
• apollo_propeller: remove HandlerOut::Unit from behaviour event path #13351
• apollo_propeller: guard inbound reads on non-empty unsent_units buffer #14533
• apollo_propeller: route inbound units through bounded channel #13350 : 1 other dependent PR (#13582 )
• apollo_propeller: buffer unsent units from partial batches #13349
• apollo_propeller: wire bounded unit channels between handler and engine #13348
• apollo_propeller: add RegisterHandler command and inbound unit SelectAll #13347
• apollo_propeller: add inbound_channel_capacity config field #13346
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[sirandreww-starkware]

Closing as superseded. The two substantive fixes in this draft — storing the poll waker to wake on on_behaviour_event, and draining the send queue with a SendError on DialUpgradeError — were split out and merged separately (#13334 and #13332). The only change not already on main is including the dropped-unit count in the outbound send-error message, which is being salvaged into a small standalone PR.