Add comprehensive tests for dashboard extractors

Implements 30 tests covering all extractor classes and the factory function:
- LatestValueExtractor: 5 tests for latest value extraction with various data shapes
- FullHistoryExtractor: 3 tests for complete buffer history extraction
- WindowAggregatingExtractor: 11 tests for windowed aggregation with different
  aggregation methods (sum, nansum, mean, nanmean, auto), time units, and edge cases
- create_extractors_from_params: 8 tests for factory function with various
  configurations and plotter specs
- UpdateExtractor interface: 3 tests verifying abstract interface compliance

All tests follow project conventions with NumPy-style docstrings, no private field
access, and proper use of scipp data structures. Tests verify observable behavior
through public interfaces only.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

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Original prompt: Please implemented tests for all the extractors in @src/ess/livedata/dashboard/extractors.py

Author: SimonHeybrock