feat: add sqlserver__openquery macro and SQL Server best-practices guide

[axellpadilla]

Adds a sqlserver__openquery helper for linked-server pass-through queries, its
tests, and a SQL Server configuration guide. 3 files, +515, no existing code
touched.

Macro

dbt/include/sqlserver/macros/utils/openquery.sql

{{ sqlserver__openquery('LOCALLOOP', "select id from RemoteDb.dbo.customers where active = 1") }}
-- OPENQUERY([LOCALLOOP], 'select id from RemoteDb.dbo.customers where active = 1')

• doubles single quotes and strips carriage returns in the remote SQL
• brackets the server name
• rejects empty arguments, and queries past the 8 KB OPENQUERY limit — the
  error reports the actual escaped length and points at EXEC(...) AT or a
  remote view

Validation is execution-phase ({% if execute %}) rather than parse-time, so one
run reports every bad model instead of aborting on the first.

Tests

tests/functional/adapter/mssql/test_openquery.py — eight models built by a
single dbt run. Three execute OPENQUERY against a LOCALLOOP loopback linked
server (MSOLEDBSQL) and assert both the emitted SQL and the rows the server
returned; the rest cover the length boundary and the validation errors. The
linked server is created and dropped inside the fixture.

Requires a live SQL Server and sp_addlinkedserver rights. Verified 9 passed
on SQL Server 2022 (16.0.4265.3), mssql-python backend.

Guide

docs/sqlserver-best-practices.md — SQL Server defaults that are wrong for a dbt
workload, each with Microsoft's recommended value and why it matters for this
adapter:

Setting	Ships as	Why it matters here
RCSI	off	Readers block behind every writer; also the documented prerequisite for table_refresh_method: dml
tempdb data files	one, 8 MB	Allocation contention under threads > 1; RCSI's version store lives here too
max server memory	unlimited	Starves the OS
MAXDOP	0	Competes with the model-level parallelism dbt already runs via threads
Recovery model	FULL	SELECT INTO, how the adapter materializes tables, is only minimally logged outside FULL

Plus OPENQUERY over 4-part names: stage remote reads as models rather than
joining across the boundary, which also avoids error 3988 under the adapter's
default XACT_ABORT ON.

A closing section covers cost threshold for parallelism and optimize for ad hoc workloads — both marked as having no published target value and unmeasured
against a dbt workload, framed as candidates to test rather than settings to
apply.

All claims cited to Microsoft Learn; adapter-specific claims link to the macro or
materialization they come from.

[axellpadilla]

@joshmarkovic , @Benjamin-Knight please review this best-practices guide