Fix and improve index.sql regression test coverage

[jrgemignani]

NOTE: This PR was created with AI tools and a human.

• Remove unused copy command (leftover from deleted agload_test_graph test)
• Replace broken Section 4 that referenced non-existent graph with comprehensive WHERE clause tests covering string, int, bool, and float properties with AND/OR/NOT operators
• Add EXPLAIN tests to verify index usage:
  • Section 3: Validate GIN indices (load_city_gin_idx, load_country_gin_idx) show Bitmap Index Scan for property matching
  • Section 4: Validate all expression indices (city_country_code_idx, city_id_idx, city_west_coast_idx, country_life_exp_idx) show Index Scan for WHERE clause filtering

All indices now have EXPLAIN verification confirming they are used as expected.

modified: regress/expected/index.out
modified: regress/sql/index.sql

[Copilot]

Pull request overview

This PR fixes broken regression tests and significantly improves index test coverage by removing obsolete code and adding comprehensive WHERE clause tests with index verification.

• Removes unused copy command leftover from deleted agload_test_graph test
• Adds EXPLAIN tests to Section 3 to verify GIN indices are used for property matching
• Replaces broken Section 4 (which referenced non-existent graph) with comprehensive WHERE clause tests covering string, integer, boolean, and float properties with various operators (AND/OR/NOT, comparison operators)

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
regress/sql/index.sql	Removed unused copy command, added GIN index EXPLAIN tests to Section 3, replaced broken Section 4 with comprehensive WHERE clause tests including expression indices and EXPLAIN verification
regress/expected/index.out	Updated expected output to match new test queries, showing proper index usage in EXPLAIN plans and correct query results

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