python(fix): support nested calculated channel references

[wei-qlu]

Summary

• Surfaces a calculated_channel: CalculatedChannel | str field on ChannelReference and routes it into the proto's calculated_channel_version_id oneof when serializing calculated channels (create + update), rules (create + update), and exports. A CalculatedChannel input is normalized to its version_id via a validator; a string is interpreted as a version_id directly.
• Skips the name-based channel lookup in resolve_calculated_channels when a version reference is already provided, so nested references don't get treated as native channel lookups and fail.
• Updates Rule._from_proto and ChannelReference._from_proto to read the version ID back off the proto oneof on the response path.

Use case:

cc = client.calculated_channels.find(name="TEST_CC")

# Reference the latest version (cc IS the latest)
ChannelReference(channel_reference="$1", calculated_channel=cc)

# Pin to a specific version_id (e.g., copied from the UI)
ChannelReference(channel_reference="$1", calculated_channel="8feb6ccd-...")

Validation

• Unit tests covering:
  • ChannelReference validation and proto round-tripping (oneof read + write), including object input, string input, and rejection cases
  • Calculated channel serialization on both Create and Update paths
  • Rules serialization helper in both directions, plus Rule._from_proto round trip
  • resolve_calculated_channels skipping the name lookup when a version reference is provided
• Manually created and updated a nested CC using the new calculated_channel field on ChannelReference and confirmed there were no missing dependents.
• Manually created a rule whose expression references a CC by version_id and confirmed it evaluated correctly.

[github-actions]

Python docs preview: https://sift-stack.github.io/sift/python/pr-580/

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