[9.0] Backport regex operand parentheses fix

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Backports #3927 (b123371) to hotfix/9.0.5 and addresses #3923.

Preserves the merged implementation: precedence-aware parentheses for both regex operands, corrected regex/concatenation precedence, and JSON traversal handling with the CAST exception. Constant, column, and parameter patterns remain unchanged; the six JSON-existence baseline updates are retained.

Only cherry-pick conflict resolution was needed for 9.0. The existing EF Core 9.0.14 JsonScalarExpression handling passes the ToJson regression without additional product changes. SDK/dependency manifests are unchanged.

Validation using isolated .NET SDK 9.0.312 and local PostgreSQL 18.4:

• Functional test project builds with zero warnings or errors.
• Focused regex tests: 44 passed.
• JSON DOM/POCO/ToJson/ad hoc JSON, Northwind functions (including string translations), array, and full-text suites: 1,338 passed, 14 pre-existing skips, zero failures.
• New regressions assert regex operators in generated SQL, including literal-option projections and synchronous/asynchronous ToJson queries.

The source commit was cherry-picked with -x; a separate empty commit records Copilot-assisted backport attribution without amending the cherry-pick.