fix(filter)!: read a quoted "null" as text for a string property

[pdevito3]

Summary

A quoted "null" on a string property is now the text null. Before this change, the parser read it as a real null. An unquoted null does not change.

 CreateRightExprFromType(type, right, rightIsQuotedLiteral, op)
-  if right == "null"
+  if right == "null" and not (rightIsQuotedLiteral and type is string)
     return null constant
   ...

Filter	Before	After
Title == "null"	x.Title == null	x.Title == "null"
Title != "null"	x.Title != null	x.Title != "null"
Title @= "null"	x.Title.Contains(null)	x.Title.Contains("null")
Title == null	x.Title == null	x.Title == null (no change)
Age == "null"	x.Age == null	x.Age == null (no change, an int has no text value)

Notes:

• This relates to fix(operators)!: reject a null value for case-sensitive string operators #117. After this change, Title @= "null" is a text search, not a null value.
• In memory, a case-sensitive @= on a row with a null property throws a NullReferenceException. That is a separate bug, and fix(operators)!: check for null in case-sensitive string operators #145 corrects it. Postgres is not affected.
• Custom operations (fix(parser)!: keep quoted custom operation values as strings #118) use ConvertStringToBasicType, which still maps a quoted "null" to null. This PR does not change that path.
• In a list, ^^ ["null"] was already the text null for a string, because the grammar removes the quotes. An unquoted [null] in a list is also text. This PR does not change the list path.

Evidence

Harness: verify-querykit, run qk-20261002-223925-97048. The seed data has no row with the text null in Directions. One row (Plain Water) has a null Directions.

• Before (main d81242c):

  • Directions == "null": expression x.Directions == null, returns Plain Water on memory and Postgres.
  • Directions != "null": expression x.Directions != null, returns 3 rows.
  • Directions @= "null": expression Contains(null), memory throws ArgumentNullException, Postgres returns [].

  After (labels with -fixed):

  • Directions == "null": expression x.Directions == "null", returns [] on both targets.
  • Directions != "null": returns all 4 rows on both targets.
  • Directions @= "null": expression Contains("null"), Postgres returns []. Memory throws the fix(operators)!: check for null in case-sensitive string operators #145 NRE on the null row.
  • Unquoted == null and != null return the same rows as before.

• Tests:

  • Unit: quoted_null_is_text_for_a_string (==, !=, @=, !@=, ==*), unquoted_null_is_null_for_a_string_not_equal, quoted_null_is_null_for_a_type_that_has_no_text, and the enumerable theory quoted_null_matches_the_text_null_and_unquoted_null_matches_null.
  • Postgres: NullFilteringTests.quoted_null_matches_the_text_null_and_unquoted_null_matches_null with ==, !=, @=, ^^ ["null"], == null, and != null. Without the fix, the ==, !=, and @= cases fail.
  • Full suites: 482 unit tests pass. 303 integration tests pass.

Merge Danger

Door: two-way

The change is one condition in the parser. A revert puts back the old behavior.

Blast Radius: consumers

This is a breaking change. A consumer that sends Prop == "null" to find null rows on a string property now gets the rows with the text null. Change the filter to Prop == null (no quotes) to find null rows.