[8.0] Backport regex operand parentheses fix

[roji]

Backports #3927 (b123371) to hotfix/8.0.12, addressing #3923 for EFCore.PG 8.0.

8.0 is affected: a JSON-derived regex pattern can concatenate the regex option prefix with the JSON document before extracting its property, producing invalid SQL behavior and PostgreSQL error 22P02.

• Preserve the merged precedence-aware handling of both regex operands, the regex/concatenation precedence correction, and the CAST exception. Constant, column, and parameter patterns remain unchanged.
• Preserve the six JSON-existence baseline parentheses introduced by the shared precedence helper.
• Adapt conflict context to the 8.0 fixtures and negated SQL expectations to 8.0's existing NOT (... ~ ...) / NOT (... ~* ...) behavior; do not introduce the newer negation optimization. EF Core 8.0.11 already handles JsonScalarExpression, so the ToJson regression needs no additional provider changes.

Verification (.NET SDK 8.0.425, PostgreSQL 18.4): functional test project builds with zero warnings/errors; all 40 focused regex tests pass. Related JsonDomQueryTest, JsonPocoQueryTest, JsonQueryNpgsqlTest, NorthwindFunctionsQueryNpgsqlTest, FunkyDataQueryNpgsqlTest, OperatorsQueryNpgsqlTest, ArrayArrayQueryTest, ArrayListQueryTest, and FullTextSearchDbFunctionsNpgsqlTest suites pass: 1,616 passed, 17 skipped, zero failures.

No dependency or SDK manifest changes.