Fix metric expr dimensions + Druid dialect; refactor build_v3 shared architecture

[shangyian]

Summary

This PR fixes dimension replacement in derived metric expressions and outputs metric combiners in Druid dialect. It also refactors the build_v3 SQL generation to use a cleaner shared architecture.

Fix: Auto-detect dimensions from metric expressions

Metrics with expressions that referenced dimensions (e.g., LAG(...) OVER (ORDER BY week)) failed to generate correct SQL because the dimension in the ORDER BY clause wasn't being replaced with its alias.

This change adds add_dimensions_from_metric_expressions() to scan combiner ASTs for dimension references and add them to ctx.dimensions before SQL generation.

Fix: Output metric_combiners in Druid dialect

The metric_expression stored in materialization config was output in Spark dialect, but since it is being materialized to Druid, most tools consuming it will need it to be in the Druid dialect.

This change updates combiners.py to use render_for_dialect(...) when generating metric_combiners.

Refactor: Clean shared architecture for SQL generation

Before this change, build_measures_sql and build_metrics_sql had duplicated setup code. The cube path and non-cube path were parallel implementations.

With this change, here is the new setup:

      build_measures_sql()                         build_metrics_sql()
               │                                            │
               ▼                                            ▼
      setup_build_context()                        setup_build_context()
               │                                            │
               ▼                                            ▼
      ┌───────────────────┐                        ┌───────────────────┐
      │   BuildContext    │                        │   BuildContext    │
      │  .metric_groups   │                        │  .metric_groups   │
      │  .decomposed_metrics                       │  .decomposed_metrics
      └───────────────────┘                        └───────────────────┘
               │                                            │
               ▼                                   ┌────────┴────────┐
      build_grain_groups()                         │                 │
               │                                   ▼                 ▼
               ▼                              if cube:           else:
      return MeasuresSQL                      build_sql_     build_grain_groups()
                                              from_cube_impl()      │
                                                   │                │
                                                   └────────┬───────┘
                                                            │
                                                            ▼
                                                   compute_metric_layers()
                                                   generate_metrics_sql()
                                                            │
                                                            ▼
                                                     return GeneratedSQL

Test Plan

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

Deployment Plan

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