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/generator.py

@@ -15,7 +15,11 @@ from semantic_layer.models import (
     ResolvedPlan,
     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 SQL fragments (measure `expr`, segment `expr`, filter,
@@ -673,9 +677,7 @@ class SqlGenerator:
             if isinstance(select_expr, exp.Alias):
                 select_expr = select_expr.this
 
-            filter_cond = sqlglot.parse_one(
-                f"SELECT {filter_sql}", read=self.dialect
-            ).expressions[0]
+            filter_cond = parse_predicate(filter_sql, self.dialect)
 
             def _make_case(inner_node):
                 return exp.Case(
@@ -1073,10 +1075,7 @@ class SqlGenerator:
 
         # AST-based rewriting for robustness
         try:
-            tree = sqlglot.parse_one(
-                f"SELECT {quote_reserved_identifiers(filter_expr)}",
-                read=self.dialect,
-            )
+            condition = parse_predicate(filter_expr, self.dialect)
 
             def _rewrite(node):
                 if isinstance(node, (exp.AggFunc, exp.Anonymous)):
@@ -1099,8 +1098,8 @@ class SqlGenerator:
                         ).expressions[0]
                 return node
 
-            transformed = tree.transform(_rewrite)
-            return transformed.expressions[0].sql(dialect=self.dialect)
+            transformed = condition.transform(_rewrite)
+            return transformed.sql(dialect=self.dialect)
         except Exception:
             logger.debug(
                 "AST-based HAVING rewrite failed for locality filter, falling back to regex: %s",
@@ -1292,9 +1291,7 @@ class SqlGenerator:
             return expr
 
         try:
-            tree = sqlglot.parse_one(
-                f"SELECT {quote_reserved_identifiers(expr)}", read=self.dialect
-            )
+            condition = parse_predicate(expr, self.dialect)
 
             changed = False
 
@@ -1309,9 +1306,9 @@ class SqlGenerator:
                         ).expressions[0]
                 return node
 
-            transformed = tree.transform(_replace)
+            transformed = condition.transform(_replace)
             if changed:
-                return transformed.expressions[0].sql(dialect=self.dialect)
+                return transformed.sql(dialect=self.dialect)
         except Exception:
             logger.debug("AST-based computed column expansion failed for: %s", expr)
 
@@ -1357,13 +1354,7 @@ class SqlGenerator:
 
         # Use AST to find and replace column references matching measure names
         try:
-            tree = sqlglot.parse_one(
-                f"SELECT * WHERE {quote_reserved_identifiers(f)}",
-                dialect=self.dialect,
-            )
-            where = tree.find(exp.Where)
-            if not where:
-                return f
+            condition = parse_predicate(f, self.dialect)
 
             changed = False
 
@@ -1388,7 +1379,7 @@ class SqlGenerator:
                         ).expressions[0]
                 return node
 
-            new_where = where.this.transform(_replace)
+            new_where = condition.transform(_replace)
             if changed:
                 return new_where.sql(dialect=self.dialect)
         except Exception: