Add $sort stage tests

[danielfrankcom]

This change adds test cases for the $sort aggregation stage, organized into files by property group:

• test_sort_basic_ordering.py: ascending/descending ordering for all BSON types, equal-value tiebreaking
• test_sort_numeric.py: cross-type numeric interleaving (int32, int64, double, Decimal128)
• test_sort_type_comparison.py: BSON type comparison order, within-type ordering rules
• test_sort_arrays.py: array sort key extraction (min for asc, max for desc), empty/nested arrays
• test_sort_key_resolution.py: compound sort left-to-right precedence, null/missing field behavior
• test_sort_field_paths.py: dot notation traversal, array-of-objects extraction, field name acceptance
• test_sort_direction_values.py: accepted sort order values (int64, double truncation, Decimal128 rounding, $meta)
• test_sort_order_errors.py: sort order type errors, range errors, error precedence
• test_sort_spec_errors.py: non-object/empty spec validation, field path validation, compound key limit, duplicate keys
• test_sort_meta_errors.py: $meta non-meta object, extra keys, and invalid value errors
• test_stages_position_sort.py: pipeline position behavior, $meta sort with prerequisite stages ($text, $geoNear)

As part of this change I also:

• Introduced build_raw_bson_doc helper for constructing BSON documents with duplicate keys
• Added new error codes and test constants used by the sort tests

Please note this change introduces similar machinery to #68, and a rebase will be required once one or the other is merged.

Ref: #34

[danielfrankcom]

Due to the new integration test changes, this PR needs #68 to be merged first. It adds the ignore_doc_order parameter to assertResult, which the integration tests require.

[danielfrankcom]

The #68 dependency was merged, so this should now be ready for final review.