fix(dbt): warn on non-portable metric semantics

[fanruan-tuco]

Summary

MetricFlow time-context semantics such as cumulative metrics, windows, time spine behavior, and offset-period inputs cannot be represented faithfully in Ossie. This change emits NON_PORTABLE_METRIC_SEMANTICS instead of silently producing a lossy conversion.

It also checks derived and ratio dependencies recursively, including cases such as:

- name: month_over_month_growth
  type: derived
  type_params:
    expr: (cur - pre) / pre
    metrics:
      - name: revenue
        alias: cur
      - name: revenue
        alias: pre
        offset_window:
          count: 1
          granularity: month

Fixes #317

Validation

uv run pytest -q in converters/dbt: 103 passed.