refactor: review ir module to enhance code readability

[tianzhou]

No description provided.

[Copilot]

Pull request overview

This pull request refactors the IR module to enhance code readability by extracting inline maps, constants, and duplicate logic into well-documented, reusable components.

Changes:

• Extracted type mapping dictionaries and constants to package-level for better maintainability and performance
• Eliminated duplicate logic between Function and Procedure argument handling
• Replaced inconsistent string conversion calls with the existing safeInterfaceToString helper

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
ir/normalize.go	Extracted postgresTypeNormalization map from function to package-level variable
ir/ir.go	Deduplicated Function/Procedure argument extraction logic into shared helper functions
ir/inspector.go	Extracted constants for trigger types and sequence max values, oidToTypeName map, and sequence max value helper; standardized string conversions

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