fix(mavlink): harden endpoint transport lifecycle

[mbuckton]

Scope

Consolidates the endpoint/network-layer fixes reviewed from:

• agent/mavlink-runtime-hardening
• feature/mavlink-transport-lifecycle-fixes

This branch starts from current development and deliberately excludes twin/state changes.

Required fixes retained

• treat UDP facade close as logical session cleanup without closing the shared UDP endpoint
• process forwarded MAVLink frames through the normal raw-frame pipeline and prevent forwarding loops
• re-register selector reads in a finally block
• clean up failed per-device facade/session creation
• isolate serial input/output scratch buffers for full-duplex traffic
• retain and cancel the heartbeat scheduled future
• avoid parsing UDP socket addresses for serial outbound messages and heartbeats
• track inbound sequence state per system/component
• guarantee connection-manager removal when protocol close partially fails
• replace broad Throwable handling/stack traces in the outbound path with bounded logging

MAVLink V1 correction

MAVLink V1 has four bytes after magic and payload length: sequence, system ID, component ID, and message ID. A complete V1 frame is therefore payload length + 8 bytes. The old serial parser and its test fixture both used a fabricated fifth header byte; both are corrected here. UDP extraction already used payload + 8.

Tests

• corrected existing V1 serial fixtures
• added back-to-back V1 extraction/truncation coverage
• added concurrent serial input/output regression coverage
• added UDP facade logical-close/idempotence coverage

Deliberately excluded

• formatting/import-only churn
• the broad runtime catch in MavlinkProtocolFactory, which logged but did not establish a complete cleanup contract
• all state/mavlink and twin management changes (separate PR)

Validation note

Focused Maven execution was attempted, but this environment cannot resolve repository.mapsmessaging.io or Maven Central. GitHub Actions is the authoritative test run for this draft.

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