Fix PostgreSQL test errors: union operations and JSON field access

[Copilot]

This PR fixes several PostgreSQL-related test failures that were preventing proper testing and usage of Red ORM with PostgreSQL databases.

Issues Fixed

1. Union/Minus Operations (t/14-union.rakutest)

• [x] Remove outdated skip statement for MINUS operations in PostgreSQL tests
• [x] Enable all 18 tests to run (previously 6 were skipped with "Pg do not accept minus")
• [x] Verify PostgreSQL driver correctly translates MINUS operations to EXCEPT syntax

2. JSON Field Access (t/37-json.rakutest)

• [x] Change PostgreSQL driver to use stringify-json = False for proper JSONB handling
• [x] Fix JSONB path syntax: properly escape both opening and closing braces to generate '{key}' instead of incomplete '{key'
• [x] Update JSON deflator to use compact format (:!pretty)

Technical Changes

• lib/Red/Driver/Pg.rakumod:
  • Fixed JSONB path operator to use '\{$path\}' syntax to generate proper '{key}' paths
  • Changed stringify-json from True to False for native JSONB storage
• t/14-union.rakutest: Removed outdated skip statement for MINUS operations
• lib/Red/Type/Json.rakumod: Updated deflator to use compact JSON format

Testing Status

• ✅ Union test: All operations enabled for PostgreSQL
• ✅ JSON test: JSONB path syntax corrected for proper field access
• ✅ Core functionality: Maintains backward compatibility

The latest fix addresses a syntax error where the closing brace wasn't properly escaped in the JSONB path generation, which would have caused malformed SQL queries.

Fixes #589.

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