`StreamableCodec` silently drops `null` elements during JSON encoding

[shaaravraghu]

Issue type
Data-loss bug

Severity
Medium

Priority
P2

Affected modules
base-transport-json

Affected files
base-transport-json/src/main/java/build/base/transport/json/codec/StreamableCodec.java

Verification status
Source inspection

Problem summary
When encoding a Streamable<T>, the codec skips null elements instead of preserving them or rejecting them. The decoded value therefore has fewer elements than the original.

Evidence
The encoder iterates elements at lines 72-75 and only writes elements when element != null.
The decoder reconstructs the stream from the encoded array at lines 95-98, so omitted null entries cannot be restored.

Relevant references
base-transport-json/src/main/java/build/base/transport/json/codec/StreamableCodec.java:72
base-transport-json/src/main/java/build/base/transport/json/codec/StreamableCodec.java:95

Why this matters
This is a lossy round-trip for collections that intentionally contain null.
Silent loss is harder to detect than a fail-fast validation error.

Reproduction steps

1. Create a marshalled type with a Streamable<String> field containing ["a", null, "b"].
2. Encode it through JsonTransport.
3. Decode it and compare element count and positions.

Expected result
The codec should either preserve null elements explicitly or reject them with a clear error.

Actual result
null elements are silently omitted from the output JSON array and disappear on decode.

Suggested fix direction
Encode null elements as JsonNull, or document and enforce a non-null-element contract with validation.

Acceptance criteria
Round-trip behavior for Streamable with null elements is deterministic and covered by tests.
The codec no longer silently changes collection cardinality.

Suggested labels
bug
data-loss
transport
json