[FEATURE] Add element_count to unexpected rows results

[BALOGUN-DAVID]

Closes #11045

Changes

• Add table.row_count as a metric dependency for UnexpectedRowsExpectation.
• Return element_count in BASIC, SUMMARY, and COMPLETE validation result payloads.
• Add regression assertions for element_count while preserving existing result_format behavior.

Tests

• .\.venv\Scripts\python.exe -m pytest tests/expectations/core/test_unexpected_rows_expectation.py -v

• .\.venv\Scripts\python.exe -m invoke lint --no-pty

• Description of PR changes above includes a link to an existing GitHub issue: Unexpected_rows_expectation doesn't give element_count in the validation results #11045

• PR title is prefixed with one of: [BUGFIX], [FEATURE], [DOCS], [MAINTENANCE], [CONTRIB], [MINORBUMP]

• Code is linted - run invoke lint (uses ruff format + ruff check)

• Appropriate tests and docs have been updated

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