feat: validate planned constraint name collisions

[Tomoscorbin]

Summary

• represent missing-table primary keys with AddPrimaryKey, giving every planned constraint creation one action form
• validate explicit PK and FK creation names across the complete accepted plan set
• reject case-insensitive collisions within one catalog and schema before execution
• preserve normal dry-run reporting and dependency blocking
• document the plan-set validation boundary and the remaining limitation for unseen external constraints

Why

Databricks uses one case-insensitive constraint-name namespace per schema across tables and constraint kinds. Per-table validation cannot detect two individually valid plans that request the same name. Without a plan-set check, one addition could succeed before a later addition failed, leaving a partially applied sync.

The previous missing-table path also embedded a primary key inside CreateTable, forcing global validation to treat new-table primary keys as a special case. Normalizing them to AddPrimaryKey keeps constraint creation uniform.

User impact

A dry run now reports planned explicit-name collisions as ConstraintNameMustBeUniqueInSchema failures. Real runs reject every affected table before executing any of its statements. Independent tables may still execute, and foreign-key dependents are blocked through the existing dependency mechanism.

Unnamed constraints and already-satisfied declarations do not participate. Collisions with unseen or externally managed catalog constraints remain Databricks execution failures.

Validation

• uv run pytest -q — 1,284 passed, 78 deselected, 97.15% coverage
• uv run ruff check .
• uv run ruff format --check .
• uv run mypy .
• uv run lint-imports
• uv run --group docs sphinx-build -W -b html docs /tmp/delta-engine-pr5-docs

[Tomoscorbin]

Superseded by #361. We decided not to add a sync-wide collision-preflight boundary: Databricks owns the authoritative schema namespace, a local check would remain incomplete, and explicit naming is optional. #361 documents the execution-time and partial-sync limitation instead.