Refactor dimension link validation into NodeSpecBulkValidator

[shangyian]

Summary

Previously, dimension link validation was scattered across the deployment orchestrator as several instance methods (e.g., validate_dimension_links). However, these methods were interleaved with deployment execution, making them hard to test in isolation and impossible to call without a running orchestrator.

What changes:

• DimensionLink.parse_join_sql and build_foreign_key_mapping become static methods. They previously required an instantiated DimensionLink ORM object (needing node_revision and dimension relationships loaded). Making them static lets validation call them with just the raw string values from a deployment spec, without touching the DB.
• NodeSpecBulkValidator gains _validate_dimension_link_specs, which runs join clause SQL parsing and column existence checks for both join and reference link specs as a post-validation pass. It prefetches dimension node columns in one bulk query (_prefetch_dimension_link_nodes) to avoid per-link DB round trips, and gracefully skips dim-side column checks for dimensions being deployed in the same batch.
• The four orchestrator validation methods are deleted. Dimension link validation now happens entirely inside bulk_validate_node_data (the pre-deployment validation step), not during execution.
• update_ast_column_types is promoted from a closure inside validate_node_data to a module-level function in internal/validation.py, so NodeSpecBulkValidator can call it after parsing to keep column type inference consistent.
• CompileContext gets a column_overrides field so that validation can substitute a proposed column list for a node (instead of loading from DB). This is a prerequisite for dry-run impact preview in later PRs. This is also threaded thru to bulk_validate_node_data
• Subscripts raises DJParseException instead of returning None when a struct field key isn't found, preventing silent type inference failures downstream.

Test Plan

• PR has an associated issue: #
• make check passes
• make test shows 100% unit test coverage

Deployment Plan

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