python(feat): expose include_received_at in get_data

[wei-qlu]

Summary

Exposes GetDataRequest.include_received_at field

• client.channels.get_data(...), get_data_as_arrow(...), and Channel.data(...) now take include_received_at
• When set, each returned DataFrame gains a "<channel_name>.sift_received_at" column with the time each point was received by Sift, NaT for points without one
• Not available for enum or bitfield channels (their protos carry no extras field)
• Received-at data is never cached, so these calls always fetch from the server

Hardening for the new nullable column in _merge_pages:

• Row-level dedup on duplicate timestamps instead of per-column groupby.last(), which skips nulls and could pair a fresh value with a stale row's received-at
• Explicit datetime64[ns, UTC] dtype so an all-NaT page doesn't degrade the merged column to object dtype

Test plan

• Extras parsing: prefixed column, NaT for unset points, foreign extras ignored, length-mismatch guard, real bitfield/enum payloads unaffected
• Request flag set vs unset on the proto, cache bypass (read and write)
• Page merge with duplicate timestamps and all-NaT pages
• Flag forwarding pinned at each layer: Channel.data, ChannelsAPI.get_data, low-level client

[github-actions]

Python docs preview: https://sift-stack.github.io/sift/python/pr-754/

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