fix(dbt): reject composite keys

[mattfaltyn]

Summary

• Reject composite primary and unique keys before Ossie-to-MSI conversion can flatten their grouped semantics.
• Preserve existing singleton primary and unique key behavior.
• Document the unsupported input and cover both composite key forms with regression tests.

This replaces plausible but incorrect MetricFlow output with an explicit ValueError until grouped keys have a lossless MSI representation.

Related Issues

Closes #311

Checklist

Specification

• No specification changes

Converters

• Converter logic is updated
• Regression tests cover composite primary and unique keys

Documentation

• The dbt conversion notes document the rejected input

Tests

• uv run --python 3.11 --frozen pytest -q — 101 passed, 5 snapshots
• uv run --frozen pytest -q on Python 3.12 — 101 passed, 5 snapshots
• uv run --python 3.14 --frozen pytest -q — 101 passed, 5 snapshots
• CLI reproduction exits nonzero and writes no manifest

Compliance

• No dependencies or new files added