revert(graphql): Revert server-side changes for ActivateMeasurementsError

[sentry]

This PR reverts the server-side changes made to activate_measurements.graphql, activate_measurements.py, and related files in graphql_api/types/mutation/mutation.py and graphql_api/types/mutation/activate_measurements/__init__.py.

During the implementation phase, it was determined that changing the ActivateMeasurementsError union type to directly use the ResolverError interface would lead to runtime issues in GraphQL type resolution, as no InterfaceType resolver is registered for ResolverError.

The root cause analysis concluded that the issue is a client-side GraphQL query error where the client attempts to query the message field directly on a union type (ActivateMeasurementsError) without using inline fragments. The GraphQL schema is correctly defined, and the fix should be applied to the client's query to conform to the GraphQL specification for union types (e.g., error { __typename ... on ResolverError { message } }).

Therefore, these server-side changes are being reverted as they do not address the root cause and would introduce new issues.

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Fixes API-EF1

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.89%. Comparing base (701e991) to head (0d3d52e).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1429   +/-   ##
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  Coverage   91.89%   91.89%           
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  Files        1325     1325           
  Lines       50868    50868           
  Branches     1626     1626           
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  Hits        46744    46744           
  Misses       3818     3818           
  Partials      306      306           

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apiunit	94.94% <ø> (ø)

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