test(protocol): correct functional test observations and diagnostics

[mbuckton]

Why

Functional CI failures include incorrect receiver counting and diagnostics that hide the point of failure. This cleanup preserves regression coverage while making remaining failures actionable.

Changes

• MQTT 5 payload-format test observes the global receiver without a competing subscription listener, checks every expected payload/property, isolates its topic and closes the client on failure.
• MQTT 5 overlap test counts incoming publications through one callback while retaining all subscription/unsubscribe expected counts; closes its client on assertion failure.
• Aggregator test correlates JSON structurally and exposes missing/malformed output and content-type errors to assertions; removes stale expected-failure comment.
• NATS fanout waits for subscriber flush before publication and remembers earlier order mismatches; improves partial-setup cleanup.
• Paho wrapper joins output readers and checks exit status, preserving subprocess failure details and existing exclusions.

Validation

• git diff --check passed.
• Java compiler parser accepted all five changed source files (syntax only, using installed JDK 17; no type checking).
• Attempted focused Maven tests with available Java 21 toolchain, but POM resolution failed before compilation: AWS SDK BOM 2.46.15 and Jersey BOM 4.0.2 unavailable due repository/DNS access. Further inspection also found the available Java 21 toolchain's module image inconsistent; it is not a reliable validation environment.
• No runtime pass claimed. Draft pending Jenkins verification.

Remaining investigation

Paho flow_control2 broken pipe, NATS flush timeout, CANAerospace/N2K ingestion and extra authentication/system-topic failures remain unresolved. No production defect is asserted from timeout evidence alone. No tests disabled or deleted; REST schema failures excluded.

Jira: https://mapsmessaging.atlassian.net/browse/MSG-263
Refs: MSG-263

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