fix(contrib/drivers/pgsql): Merge duplicated fields, especially for key constraints.

[wangle201210]

pgsql 执行TableFields 或者字段信息时需要合并key信息

[Copilot]

Pull Request Overview

This PR implements proper merging of key information for PostgreSQL table fields when multiple constraints exist on the same field. The changes ensure that when a field has multiple constraints (e.g., both primary key and unique), the TableFields method correctly prioritizes and merges the key information with the hierarchy: pri > uni > others.

Key changes:

• Modified field processing logic to merge duplicate field entries instead of skipping them
• Implemented constraint priority system where primary key takes precedence over unique constraints
• Added comprehensive test coverage for both single and composite constraint scenarios

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
contrib/drivers/pgsql/pgsql_table_fields.go	Updated field processing logic to merge key constraints with proper priority handling
contrib/drivers/pgsql/pgsql_z_unit_db_test.go	Added test cases to verify correct merging of field constraints and composite primary keys

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