feat: add UDF function resource support and adapter tests for dbt-core v1.11

[axellpadilla]

Summary

Adds UDF (user-defined function) resource support and adapter tests for dbt-core v1.11's new function resource type (issue #761, part of #719).

Changes

• Added Function = "function" to SQLServerRelationType enum

New: dbt/include/sqlserver/macros/materializations/functions/scalar.sql

SQL Server-specific scalar UDF macros:

• sqlserver__scalar_function_sql — main entry point
• sqlserver__scalar_function_create_replace_signature_sql — uses CREATE OR ALTER FUNCTION with 2-part names (SQL Server forbids database prefix)
• sqlserver__formatted_scalar_function_args_sql — @param type syntax with default value support
• sqlserver__scalar_function_body_sql — BEGIN RETURN ... END
• sqlserver__scalar_function_volatility_sql — no-op (SQL Server has no volatility keywords)
• sqlserver__unsupported_volatility_warning — warns when volatility is set

New: dbt/include/sqlserver/macros/materializations/functions/helpers.sql

• sqlserver__function_execute_build_sql — runs USE [db] in a separate batch before CREATE FUNCTION (required because CREATE FUNCTION must be the first statement in a batch)

New: tests/functional/adapter/dbt/test_functions.py

15 test classes (13 active, 2 skipped for UDAFs):

• TestUDFsBasic — scalar UDF build + execute
• TestDeterministicUDF, TestStableUDF, TestNonDeterministicUDF — volatility warnings
• TestErrorForUnsupportedType — type: table fails gracefully
• TestPythonUDFNotSupported — Python UDFs fail gracefully
• TestSqlUDFDefaultArgSupport — default parameter values
• TestUDFParse — dbt parse with functions/
• TestUDFList — dbt list --select resource_type:function
• TestUDFBuild — dbt build --select resource_type:function
• TestUDFModelRef — model using {{ function('name') }}
• TestUDFSchemaCreation — schema created before function
• TestUDFRebuild — function rebuild via CREATE OR ALTER
• TestBasicSQLUDAF, TestBasicPythonUDAF — skipped (SQL Server doesn't support aggregate functions)

SQL Server-Specific Adaptations

Issue	Solution
3-part names forbidden in CREATE FUNCTION	USE [db] in separate batch, then 2-part name
CREATE FUNCTION must be first statement	Separate USE batch via function_execute_build_sql override
No IMMUTABLE/STABLE/VOLATILE	Emit nothing + warning
Parameters require @ prefix	Override formatted_scalar_function_args_sql
Default params use = value syntax	Override formatted_scalar_function_args_sql
Function body uses BEGIN...END	Override scalar_function_body_sql
Calling with defaults requires DEFAULT keyword	Test uses func(DEFAULT) not func()

Testing

All 13 active tests pass against a live SQL Server instance. 2 UDAF tests skipped with explicit reason.