`Connection` uses `null` as a publish-stream shutdown sentinel and makes `null` payloads impossible

[shaaravraghu]

Issue type
Public API bug

Severity
Medium

Priority
P2

Affected modules
base-network

Affected files
base-network/src/main/java/build/base/network/Connection.java
base-flow/src/main/java/build/base/flow/Producer.java

Verification status
Source inspection

Problem summary
The publish path treats null as an internal control signal meaning "producer closed." Because of that, callers cannot publish a legitimate null payload even though Producer.publish(T item) does not document null as invalid.

Evidence
publish marks the producer non-operational and removes it when item == null at lines 197-202.
Producer.close() delegates to publish(null) at lines 237-240.
The receiving side treats item == null as stream completion at lines 387-390.

Relevant references
base-network/src/main/java/build/base/network/Connection.java:197
base-network/src/main/java/build/base/network/Connection.java:238
base-network/src/main/java/build/base/network/Connection.java:387
base-flow/src/main/java/build/base/flow/Producer.java:35

Why this matters
The transport conflates control signals with data.
That makes the wire contract surprising and can silently terminate a publish stream when a caller sends null.

Reproduction steps

1. Create a producer through Connection.createProducer.
2. Call publish(null).
3. Observe from the code path that the producer is removed locally and the subscriber registry completes remotely.

Expected result
If null payloads are invalid, the API should reject them explicitly.
If null payloads are valid, shutdown should use a distinct control frame rather than overloading payload data.

Actual result
null is silently interpreted as "close the producer."

Suggested fix direction
Use a dedicated operation or frame for producer completion, or reject null with a clear exception at the API boundary.

Acceptance criteria
Publish stream shutdown is represented separately from payload data.
Producer.publish null-handling is documented and tested.

Suggested labels
bug
network
api-behavior