feat: Support incompatible type conversions via DROP/ADD approach

[bheemreddy181]

Summary

Resolves #179 by implementing intelligent type coercion analysis that automatically switches to DROP COLUMN + ADD COLUMN for incompatible type conversions instead of failing with cast errors.

Description

This PR introduces a comprehensive type coercion analysis system that intelligently handles column type changes by categorizing conversions and choosing the appropriate alteration strategy:

• Binary coercible types (e.g., smallint → integer): Use ALTER COLUMN with minimal locking
• Coercible types (e.g., text → integer): Use ALTER COLUMN with table rewrite warnings
• Incompatible types (e.g., integer → timestamptz): Use DROP COLUMN + ADD COLUMN with data loss warnings

Key Changes

• Added GetTypeCoercionInfo() function for comprehensive type conversion analysis
• Enhanced column alteration logic to automatically select optimal conversion approach
• Improved hazard reporting to distinguish between different conversion methods
• Added appropriate data loss warnings for destructive operations
• Maintains full backward compatibility

Motivation

Problem: Previously, attempting to alter incompatible column types (like integer to timestamptz) would fail with cast errors, requiring manual intervention.

Solution: This change automatically detects incompatible conversions and uses the appropriate DROP/ADD strategy while providing clear warnings about potential data loss.

Related Issues:

• Fixes [FEATURE] Support switching between non-coercible types via drop/add #179 (main issue)
• Addresses Don't include ACQUIRES_ACCESS_EXCLUSIVE lock migration hazard if switching between binary-coercible types #52 for improved hazard reporting on binary coercible types

Technical Details

The implementation categorizes type conversions into three levels:

1. Binary Coercible: Direct type casting possible, minimal performance impact
2. Coercible: Casting possible but may require table rewrite
3. Incompatible: No direct casting possible, requires column recreation

Testing

New Test Coverage

• Comprehensive unit tests for type coercion logic
• Integration tests covering all [FEATURE] Support switching between non-coercible types via drop/add #179 scenarios
• Edge case testing for various type combinations

Regression Testing

• All existing tests continue to pass
• Backward compatibility verified across all scenarios

Before/After

Before:

ALTER COLUMN integer_col TYPE timestamptz;
-- ❌ Fails with: "cannot cast type integer to timestamp with time zone"

After:

-- ✅ Automatically generates:
-- DROP COLUMN integer_col;
-- ADD COLUMN integer_col timestamptz;
-- With appropriate hazard warnings about data loss

[bheemreddy181]

@bplunkett-stripe thoughts ?

[bheemreddy181]

@Navbryce can I get a code review when ever you can ?

[bheemreddy181]

@Navbryce @bplunkett-stripe can you take a look at this and share some feedback ?