fix: handle IB formatted timestamps and optional realtime integer fields

[faysou]

Summary

• Added timezone-aware parsing for IB response timestamps in src/messages.rs, covering Unix timestamps, UTC-formatted yyyyMMdd-HH:mm:ss values, timestamps with embedded timezone names, session-local yyyyMMdd HH:mm:ss values, and date-only yyyyMMdd values when a session timezone is available.
• Threaded decoder timezone context through head-timestamp and realtime market-data decoders so formatted IB timestamps can be interpreted correctly in both sync and async paths.
• Tightened realtime stream handling by explicitly surfacing IncomingMessages::Error for realtime bars and by switching several decoder integer reads to message.next_optional_int()?.unwrap_or_default(), so omitted fields such as bar counts, tick attribute masks, market-depth positions, operations, and sides default to zero instead of failing on empty fields.
• Expanded unit coverage for the new timestamp parsing rules and updated realtime-bar expectations to match UTC-formatted IB timestamps.

Design notes

• UTC-formatted IB timestamps now decode as UTC regardless of the session timezone, which avoids offset-shifting values like 20260328-12:34:56.
• Session-local parsing still uses the connected TWS/Gateway timezone, including rejection of ambiguous or nonexistent local times during DST transitions.
• Date-only parsing is evaluated before the Unix-seconds fallback when session timezone context exists, preventing YYYYMMDD values from being misread as 1970-era epoch seconds.
• Optional integer decoding now treats empty IB fields as unset and falls back to zero where the wire protocol leaves those values blank, which makes realtime and market-depth decoders more tolerant of incomplete messages without changing the decoded shape.

Testing

• cargo test --features sync
• cargo test --features async
• cargo fmt --all

[faysou]

should I do the PR on top of v2-stable instead ?

[wboayue]

Thanks!