Revert: cross join guardrail back to UserWarning (not ValidationError)

Principle: SDK should not be more restrictive than the server.
The server allows cartesian queries (FROM a, b) -- they work and
return results (capped at 5000). The SDK warns users but does not
block, because:
1. Small-table cartesian products are safe (3 rows * 3 rows = 9)
2. Server has its own resource governance (5000-row cap, timeouts)
3. Blocking what the server allows creates user frustration

Final guardrail design:
- ValidationError: INSERT/UPDATE/DELETE (server blocks too, but SDK
  catches earlier with clearer message)
- UserWarning: FROM a, b (cartesian), LIKE '%value', SELECT * + JOIN

756 unit tests passing.

Author: Saurabh Badenkal

src/PowerPlatform/Dataverse/data/_odata.py

@@ -927,15 +927,17 @@ def _sql_guardrails(self, sql: str) -> str:
                 stacklevel=4,
             )
 
-        # 3. Block implicit cross joins
+        # 3. Warn on implicit cross joins (server allows these but they
+        # produce cartesian products that can stress shared DB resources)
         if self._SQL_IMPLICIT_CROSS_JOIN_RE.search(sql):
-            raise ValidationError(
-                "Implicit cross join detected (FROM table1, table2). "
+            warnings.warn(
+                "Query uses an implicit cross join (FROM table1, table2). "
                 "This produces a cartesian product that can generate "
                 "millions of intermediate rows and degrade shared database "
                 "performance. Use explicit JOIN...ON syntax instead: "
                 "FROM table1 a JOIN table2 b ON a.column = b.column",
-                subcode=VALIDATION_SQL_CROSS_JOIN_BLOCKED,
+                UserWarning,
+                stacklevel=4,
             )
 
         return sql

tests/unit/data/test_sql_guardrails.py

@@ -156,30 +156,38 @@ def test_no_like_no_warning(self):
 
 
 # ===================================================================
-# 4. Block implicit cross joins
+# 4. Warn on implicit cross joins (server allows, SDK warns)
 # ===================================================================
 
 
-class TestImplicitCrossJoinBlocked:
-    """FROM a, b (comma syntax) must be blocked with ValidationError."""
+class TestImplicitCrossJoinWarning:
+    """FROM a, b (comma syntax) should emit UserWarning (not error)."""
 
-    def test_comma_join_raises(self):
+    def test_comma_join_warns(self):
         c = _client()
-        with pytest.raises(ValidationError, match="cross join"):
+        with warnings.catch_warnings(record=True) as w:
+            warnings.simplefilter("always")
             c._sql_guardrails("SELECT TOP 10 a.name, c.fullname FROM account a, contact c")
+            cross_warnings = [x for x in w if "cross join" in str(x.message).lower()]
+            assert len(cross_warnings) == 1
 
-    def test_explicit_join_not_blocked(self):
+    def test_explicit_join_no_warning(self):
         c = _client()
-        # Should NOT raise
-        result = c._sql_guardrails(
-            "SELECT TOP 10 a.name FROM account a " "JOIN contact c ON a.accountid = c.parentcustomerid"
-        )
-        assert "JOIN" in result
+        with warnings.catch_warnings(record=True) as w:
+            warnings.simplefilter("always")
+            c._sql_guardrails(
+                "SELECT TOP 10 a.name FROM account a " "JOIN contact c ON a.accountid = c.parentcustomerid"
+            )
+            cross_warnings = [x for x in w if "cross join" in str(x.message).lower()]
+            assert len(cross_warnings) == 0
 
-    def test_single_table_not_blocked(self):
+    def test_single_table_no_warning(self):
         c = _client()
-        result = c._sql_guardrails("SELECT TOP 10 name FROM account")
-        assert "SELECT" in result
+        with warnings.catch_warnings(record=True) as w:
+            warnings.simplefilter("always")
+            c._sql_guardrails("SELECT TOP 10 name FROM account")
+            cross_warnings = [x for x in w if "cross join" in str(x.message).lower()]
+            assert len(cross_warnings) == 0
 
 
 # ===================================================================