Fix slice panic in PolledClient.GetMessages for out-of-range start/count

[rootkiller6788]

Problem

The /polled_connections/{connId}/messages HTTP endpoint parses start and count from the request query string and passes them to PolledClient.GetMessages without any range validation. A negative start (e.g. ?start=-1) or a start beyond the number of buffered messages (e.g. ?start=100) caused make([]interface{}, count) or c.messages[start:end] to panic with a slice out-of-range error, terminating the request handler.

Change

Clamp start to [0, len(messages)] and count to the remaining messages (len(messages)-start) in GetMessages, so the method returns a sensible slice for any input:

• start < 0 is treated as 0
• start > len(messages) returns an empty slice
• count < 0 (meaning "all remaining") or count larger than the remaining messages is clamped to the remaining count

This also avoids the start + count integer-overflow edge case for very large count values by using len(messages)-start instead of computing end := start + count.

Verification

• Added TestGetMessagesBounds covering negative start, start past the end (with and without count), and an oversized count.
• Existing TestMessages behavior is preserved.