[10.0] Backport regex operand parentheses fix

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Backports #3927 (b123371) to hotfix/10.0.4, addressing #3923 for EFCore.PG 10.0. The backport commit retains the source author and cherry-pick attribution.

Preserves the merged implementation: precedence-aware parentheses for both regex operands, regex/concatenation precedence of 1000, JSON traversal handling with the CAST exception, unchanged constant/column/parameter regex patterns, and the six JSON-existence baseline updates. The only conflict resolution was placing the new JsonQueryNpgsqlTest regression without importing newer, unrelated tests. No product-code adaptation or SDK manifest change was needed.

Also updates the build-only Microsoft.SourceLink.GitHub dependency from 10.0.200 to 10.0.303 in a separate commit, as requested, to resolve the transitive Microsoft.Build.Tasks.Git advisory (GHSA-23fw-v26w-5fgq) that blocked CI restore. No audit warnings are suppressed.

Validation

Using .NET SDK 10.0.300 and the local PostgreSQL instance:

• Focused regex tests: 76 passed, including all 17 new regression cases. SQL assertions confirm server-side regex execution, including the ToJson regression and literal-options switch cases.
• Related suites (JsonDomQueryTest, JsonPocoQueryTest, JsonQueryNpgsqlTest, StringTranslationsNpgsqlTest, NorthwindFunctionsQueryNpgsqlTest, ArrayArrayQueryTest, ArrayListQueryTest, FullTextSearchDbFunctionsNpgsqlTest): 988 passed, 10 existing skips, 0 failures. The substring filter also includes AdHocJsonQueryNpgsqlTest.
• After the SourceLink update: dotnet build --configuration Debug succeeded with 0 warnings and 0 errors, without the warning override; database-free unit tests had 489 passed, 6 existing skips, 0 failures.

The database-backed runs preceded the SourceLink update and used the command-line-only -p:WarningsNotAsErrors=NU1902 override for the then-existing advisory. The dependency update removes that restore blocker.