feat(sqlserver): Part 4 — relation quoting and factory wiring

[axellpadilla]

Part of dbt-labs#15714 (upstream feature request). Depends on #1. Independent of #5, #6, #7 (can run in parallel).

Work items

• src/relation/relation_impl.rs — include_policy match, db/schema/identifier string-formatting arms, constructor (mirror Fabric's arms)
• src/relation/factory.rs — create_static_relation match

Full detail: dbt-sqlserver-v2-roadmap plan/02-implementation-steps.md §5.5 ("Relation type & quoting", "Relation factory")

PRs against sqlserver-v2-port in this repo, use Closes #<this issue>.