fix: skip empty SSE data lines to avoid spurious parse errors

[lewis617]

Problem

app.log was flooded with errors every ~12s:

[ERROR] Failed to parse stream chunk {"error":{},"data":""}

Root Cause

Servers/proxies send empty data: lines as SSE keepalive heartbeats. The streaming parser in openaiClient.ts attempted JSON.parse("") on these empty payloads, throwing a SyntaxError each time. The SyntaxError object's message property is non-enumerable, so it serialized as {} in the log.

Fix

• Skip empty data lines (if (!data) continue) before attempting JSON.parse, silently ignoring keepalive heartbeats.
• Log actual error message (e instanceof Error ? e.message : String(e)) instead of the raw error object, so any future parse failures are diagnosable.

Testing

• All 14 existing openaiClient tests pass.
• Type-check and lint pass (pre-commit hooks verified).