[JDBC-V2, Client-V2] fixes problem with duplicate column names

[chernser]

Summary

• Fixed problem with duplicate column names
• Last duplicate column index is remembered in index-by-name map.
• Table Schema will return list of all columns anyway

Closes #2459
Closes #2336

Checklist

Delete items not relevant to your PR:

• Closes #
• Unit and integration tests covering the common scenarios were added
• A human-readable description of the changes was provided to include in CHANGELOG
• For significant changes, documentation in https://github.com/ClickHouse/clickhouse-docs was updated with further explanations or tutorials

[windsurf-bot]

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[windsurf-bot]

The table names in the DROP statements don't match the CREATE statements. The DROP statements use test_duplicate_column_names_1 and test_duplicate_column_names_2 (with underscores), but the CREATE statements use test_duplicate_column_names1 and test_duplicate_column_names2 (without underscores).

Suggested change

	client.execute("DROP TABLE IF EXISTS test_duplicate_column_names_1").get().close();
	client.execute("DROP TABLE IF EXISTS test_duplicate_column_names_2").get().close();
	client.execute("DROP TABLE IF EXISTS test_duplicate_column_names1").get().close();
	client.execute("DROP TABLE IF EXISTS test_duplicate_column_names2").get().close();

Also, consider adding similar DROP statements at the end of the test to clean up the created tables.

[windsurf-bot]

This change addresses the duplicate column names issue by keeping the last occurrence of a column name in the index map. Consider adding a comment explaining this behavior, as it's an important detail for users of this API to understand that when duplicate column names exist, only the last one will be accessible via name-based lookups.

[sonarqubecloud]

Quality Gate failed

Failed conditions
0.0% Coverage on New Code (required ≥ 80%)

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