Fix tests that violated extractor type constraints

The cleanup commit (88e7c72) simplified LatestValueExtractor to assume
all input data are scipp DataArrays, removing the defensive hasattr check.
However, several tests were passing non-DataArray types (plain ints/strings),
causing AttributeErrors.

Rather than revert the cleanup, fix the tests to respect the extractor's
type contract by storing proper scipp DataArrays:

- data_service_test.py: Updated 38 tests to use make_test_data() helper
  to create DataArrays instead of plain integers. Fixed assertions to
  extract scalar values (e.g., value.value instead of value).

- helpers_test.py: Updated 4 integration tests to use make_test_result()
  helper for storing result data as DataArrays with time coordinates.

- temporal_buffer_manager_test.py: Updated test to check for KeyError
  without regex pattern matching.

All 1451 tests now pass.

Original request: Please look into the failing tests. Did my latest cleanup commit cause this?

🤖 Generated with Claude Code

Co-Authored-By: Claude <[email protected]>

Author: SimonHeybrock