Nonpositive network batch sizes are accepted

### Feature Summary

Root cause: `NetworkOutputBuffer` accepts a batch size of zero or less. Its size check then becomes true after every tuple, so invalid API input silently changes buffering into one message per tuple.

Before: A nonpositive batch size is accepted and every tuple is sent immediately.

Expected: Construction rejects a nonpositive batch size.

Reproduction evidence:

1. Check out main at commit `d9c0e19aeed027c4a61d746d875a8b46dd1db568`.
2. Run the `NetworkOutputBufferSpec` suite.
3. Observe that the characterization test accepts zero and negative sizes while the rejection test is pending.

The runtime suite completed with 17 successful tests and 1 pending test.

Version and commit evidence:

Main, version 1.3.0 incubating snapshot.

**Commit Hash (Optional)**
`d9c0e19aeed027c4a61d746d875a8b46dd1db568`

## Browsers

Not applicable. This is an Amber network buffering issue.

## Relevant log output

```text
Total number of tests run: 17
Tests: succeeded 17, failed 0, canceled 0, ignored 0, pending 1
```

### Proposed Solution or Design

Expected: Construction rejects a nonpositive batch size.

### Affected Area

Workflow Engine (Amber)