Wire Mqtt OBS5004 member shape and OBS5005 Reactive diagnostics

[Skymly]

Summary

• Emit OBS5004 for Mqtt boundary attribute / member shape mismatches
• Emit OBS5005 via shared ObservableReturnTypeParser when R3 generator sees IObservable<T>

Test plan

• Generator tests OBS5004 / OBS5005
• CI build-test + pack

Closes #62

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Note

Low Risk
Changes are confined to compile-time source generator diagnostics and tests; no runtime MQTT or API behavior changes.

Overview
The MQTT source generator now reports OBS5004 when publish/subscribe attributes are on the wrong member kind: [MqttSubscribe] on a method or [MqttPublish] on a property, instead of treating those as generic invalid members.

Observable return-type validation in Parser is routed through shared ObservableReturnTypeParser, so the R3 generator can emit OBS5005 when an interface uses IObservable<T> without the Reactive package reference, alongside existing unsupported-return handling.

R3 generator tests assert OBS5004 and OBS5005 appear in the diagnostic snapshot for those scenarios.

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