Use precedence-aware parentheses for regex operands

[roji]

Fixes #3923.

Use RequiresParentheses for both regex operands, including the Singleline fast path and negated/case-insensitive matches. Unlike #3924, this preserves the existing SQL for constant, column, and parameter patterns instead of wrapping every pattern.

• Correct regex and concatenation precedence to PostgreSQL's generic-operator category.
• Make JSON traversal participate in parentheses decisions while preserving existing CAST handling.
• Add regression coverage for JSON DOM and ToJson operands, nested JSON paths, concatenation, and regex options.
• Update the six JSON-existence SQL baselines affected by shared traversal grouping.

[Copilot]

Copilot review overview

🟢 Approval recommended

The precedence changes are consistent with PostgreSQL parsing rules and are covered across the affected regex and JSON scenarios.

Review effort: Balanced
Findings: None

What changed in this PR

Updates SQL generation to preserve PostgreSQL operator precedence for regex and JSON traversal expressions.

Changes:

• Adds precedence-aware parentheses for regex operands and concatenation.
• Integrates JSON traversal into operator precedence decisions.
• Adds regression tests and updates affected SQL baselines.

File	Description
src/​EFCore.PG/​Query/​Internal/​NpgsqlQuerySqlGenerator.cs	Implements precedence-aware SQL generation.
test/​EFCore.PG.FunctionalTests/​Query/​NorthwindFunctionsQueryNpgsqlTest.cs	Tests concatenated regex operands.
test/​EFCore.PG.FunctionalTests/​Query/​JsonQueryNpgsqlTest.cs	Tests JSON-mapped regex operands.
test/​EFCore.PG.FunctionalTests/​Query/​JsonPocoQueryTest.cs	Updates JSON-existence baselines.
test/​EFCore.PG.FunctionalTests/​Query/​JsonDomQueryTest.cs	Adds comprehensive JSON regex regression coverage.

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