Optimize DJ SQL and fix Trino compatibility

[shangyian]

Summary

1. Optimizes /djsql/ endpoint by removing redundant validation. Validation is now delegated to build_metrics_sql(), avoiding duplicate work.
2. Made dialect parameter optional in both /djsql/ endpoint and build_metrics_sql() rather than defaulting to Spark when not specified. DJ will try to infer an appropriate dialect based on the catalogs of the available tables.
3. Fixes Trino compatibility issue with single-argument COALESCE (Trino requires COALESCE to have at least 2 arguments), but we can detect and only wrap columns in COALESCE when there are multiple CTEs.
4. Added filter_dimensions to track dimensions referenced only in where clauses. These dimensions are joined for filter resolution but excluded from output projection. Filters referencing filter-only dimensions are applied in grain group CTEs only (not in final SELECT).

Test Plan

Updated expected SQL in tests to remove single-argument COALESCE + added TestFilterOnlyDimensions test class for filter-only dimension behavior.

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

Deployment Plan

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