Implement EF.Function.JsonPathExists

[roji]

Closes #3733

[Copilot]

Pull request overview

This PR implements support for EF.Functions.JsonPathExists() in the Npgsql EF Core provider. The new EF Core API (RelationalDbFunctionsExtensions.JsonPathExists) is translated to the PostgreSQL JSON_EXISTS() SQL function (a SQL/JSON standard function introduced in PostgreSQL 16), with special handling for scalar string columns, complex JSON properties, and owned JSON entities.

Changes:

• Added translation of EF.Functions.JsonPathExists() to JSON_EXISTS() SQL in the visitor, with pattern matching for different JSON argument shapes (scalar, complex, owned entity)
• Updated three placeholder tests from AssertTranslationFailed to actual SQL assertions verifying the JSON_EXISTS(...) output
• Simplified three existing test call sites by using the extension method syntax (EF.Functions.JsonExists(...)) instead of the fully qualified static call

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/EFCore.PG/Query/Internal/NpgsqlSqlTranslatingExpressionVisitor.cs	Core implementation: new switch case translates EF.Functions.JsonPathExists() to the SQL JSON_EXISTS() function
test/EFCore.PG.FunctionalTests/Query/Translations/JsonTranslationsNpgsqlTest.cs	Replaces AssertTranslationFailed stubs with actual SQL assertions for the three JsonPathExists test variants
test/EFCore.PG.FunctionalTests/Query/JsonStringQueryTest.cs	Style cleanup: uses extension method syntax for JsonExists call
test/EFCore.PG.FunctionalTests/Query/JsonPocoQueryTest.cs	Style cleanup: uses extension method syntax for JsonExists call
test/EFCore.PG.FunctionalTests/Query/JsonDomQueryTest.cs	Style cleanup: uses extension method syntax for JsonExists call

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[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated no new comments.

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