fix: support ISO 8601 timestamp format in Packet.sniff_time for tshark 4.6+ JSON/EK output

[Sug2077]

Summary

tshark 4.6.0 changed the JSON/EK output format of frame.time_epoch from a float-based Unix timestamp to an ISO 8601 string (e.g. 2025-10-22T02:20:33.634243780Z).
This caused ValueError: could not convert string to float when parsing packets with PyShark.

https://www.wireshark.org/docs/relnotes/wireshark-4.6.0

Absolute time fields, regardless of field display in the Packet Details, are always written in ISO 8601 format in UTC with -T json. This was already the case for -T ek since version 4.2.0. JSON is primarily a data interchange format read by software, so a standard format is desirable.

Fix

Updated Packet.sniff_time to detect ISO 8601 timestamps (ending with Z) and parse them using datetime.fromisoformat().
The old float-based parsing is kept for backward compatibility.

Compatibility

• Works with both tshark ≤4.4 (float) and 4.6+ (ISO 8601)

• No external dependencies added

Fixes #718