Fix: Remove source/activities from RestSessionResource

[davideast]

This PR aligns the RestSessionResource interface with the v1alpha API schema by removing source, activities, and generatedFiles fields.

Changes:

• Modified packages/core/src/types.ts: Removed the fields and added [key: string]: unknown to support existing tests that inject these fields in mock objects.
• Modified packages/core/src/mappers.ts: Updated mapRestSessionToSdkSession to explicitly set source, activities, and generatedFiles to undefined and removed mapping logic for these fields.
• Modified packages/core/tests/mappers.test.ts: Added a verification test case to ensure these fields are correctly set to undefined.

This ensures the SDK correctly handles the sparse Session resource returned by the API.

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PR created automatically by Jules for task 13533407257037680422 started by @davideast

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