fix(streaming): improve error handling for timestamp and JSON operations

[echobt]

Problem

In src/cortex-app-server/src/streaming.rs, there were two consecutive .unwrap() calls around lines 513-516:

1. .duration_since(std::time::UNIX_EPOCH).unwrap() - for timestamp calculation
2. .unwrap() on serde_json::to_string(...) - for JSON serialization

While these operations rarely fail in practice, using .unwrap() directly can cause panics if unexpected conditions occur (e.g., system clock issues or serialization edge cases).

Solution

Replaced both .unwrap() calls with .unwrap_or_default():

• For the duration calculation: returns Duration::default() (zero duration) if the system time is before UNIX epoch
• For JSON serialization: returns an empty string if serialization fails

This is appropriate because:

1. The result is already being discarded with let _ = tx.send(...)
2. This is a non-critical ping event used only for initial connection readiness
3. Graceful degradation is preferred over panicking in production

Changes

• src/cortex-app-server/src/streaming.rs: Changed .unwrap() to .unwrap_or_default() for both the timestamp duration and JSON serialization

Testing

• Verified compilation with cargo check -p cortex-app-server

[echobt]

Consolidated into #73 - fix: consolidated error handling improvements