fix(sl): parse user filter expressions as predicates, not projections (#307)

* fix(sl): parse user filter expressions as predicates, not projections

User-authored filters and segments were parsed in a projection context
(`SELECT {expr}`). On T-SQL a top-level `col = 'value'` projection is the
`alias = expression` aliasing syntax, so an equality filter parsed this way
became `'value' AS col` — dropping the comparison entirely and silently
skipping computed-column expansion (the column hid behind the alias).

Parse user fragments as predicates (`SELECT * WHERE {expr}`) at every parse
site — the parser cache, measure-filter CASE WHEN generation, computed-column
expansion, and measure-filter/segment column qualification. For plain
non-condition expressions the column set is identical, so this is a no-op
everywhere except the T-SQL alias case it fixes.

Add cross-dialect regression tests (tsql, postgres, snowflake, bigquery)
locking equality filters/segments to comparison shape and confirming `= 'x'`
now matches `IN ('x')` on T-SQL.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* Shorten T-SQL predicate comments

* docs(sl): tighten T-SQL predicate docstrings and AGENTS docstring rule

Trim the parser and regression-test docstrings to the 1-3 line bar and
extend the AGENTS.md comment guidance to cover docstrings explicitly.

* refactor(sl): route all filter parsing through parse_predicate

Consolidate the predicate-context parse into a single parse_predicate
helper and route every filter-parsing call site through it: measure
CASE-WHEN filters, segments, computed-column-in-filter, the
aggregate-locality HAVING rewrite, and the planner OR-mixing /
top-level-AND split. The locality and split paths still parsed user
filters in projection context, so a named-measure equality filter
compiled to `0 AS measure` on T-SQL. Add a locality regression test
covering the HAVING rewrite path.

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
Co-authored-by: Andrey Avtomonov <andreybavt@gmail.com>

python/ktx-sl/semantic_layer/planner.py

@@ -22,7 +22,11 @@ from semantic_layer.models import (
     SemanticQuery,
     SourceDefinition,
 )
-from semantic_layer.parser import ExpressionParser, quote_reserved_identifiers
+from semantic_layer.parser import (
+    ExpressionParser,
+    parse_predicate,
+    quote_reserved_identifiers,
+)
 
 # DIALECT CONVENTION:
 #   User-authored measure `expr`, `filter`, and computed-column fragments must
@@ -910,9 +914,7 @@ class QueryPlanner:
         for c in source.columns:
             col_to_source[c.name] = source_name
 
-        tree = sqlglot.parse_one(
-            f"SELECT {quote_reserved_identifiers(expr)}", read=self.dialect
-        )
+        condition = parse_predicate(expr, self.dialect)
 
         def _qualify_column(node):
             if (
@@ -926,8 +928,8 @@ class QueryPlanner:
                 )
             return node
 
-        transformed = tree.transform(_qualify_column)
-        return transformed.expressions[0].sql(dialect=self.dialect)
+        transformed = condition.transform(_qualify_column)
+        return transformed.sql(dialect=self.dialect)
 
     def _detect_fan_out(
         self,
@@ -1254,14 +1256,7 @@ class QueryPlanner:
     ) -> None:
         """Raise an error if an OR expression mixes WHERE and HAVING conditions."""
         try:
-            tree = sqlglot.parse_one(
-                f"SELECT * WHERE {quote_reserved_identifiers(clause)}",
-                dialect=self.dialect,
-            )
-            where = tree.find(exp.Where)
-            if not where:
-                return
-            inner = where.this
+            inner = parse_predicate(clause, self.dialect)
             # Only check if the top level contains OR
             or_parts: list[str] = []
 
@@ -1295,14 +1290,7 @@ class QueryPlanner:
     def _split_top_level_and(self, expr: str) -> list[str]:
         """Split a filter expression on top-level AND (not inside parentheses or strings)."""
         try:
-            tree = sqlglot.parse_one(
-                f"SELECT * WHERE {quote_reserved_identifiers(expr)}",
-                dialect=self.dialect,
-            )
-            where = tree.find(exp.Where)
-            if not where:
-                return [expr]
-            inner = where.this
+            inner = parse_predicate(expr, self.dialect)
             parts: list[str] = []
 
             def _collect_and(node):