Implement missing Jules API types

[arran4]

This PR implements missing data types from the Jules API reference (https://jules.google/docs/api/reference/types). Specifically, it adds response types for listing activities and request/response types for session actions (sending messages and approving plans). Verification tests are included.

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PR created automatically by Jules for task 5541557627152087561 started by @arran4

[google-labs-jules]

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[arran4]

@jules can we think about how these are displayed and what information we need to extract from the map to make it easier to display that content.

[google-labs-jules]

@jules can we think about how these are displayed and what information we need to extract from the map to make it easier to display that content.

Excellent point. I've updated the Activity model to explicitly include local state fields (isPending, isQueued, processingErrors, etc.) that were previously being stuffed into unmappedProps. This makes the display logic in ActivityItem and ActivityHelper much cleaner and type-safe.

[arran4]

LGTM