fix limited-distinct identity contract

[shangyian]

Summary

A metric defined as COUNT(DISTINCT id) carries a derived expression like count(DISTINCT id_distinct_HASH) (it references the hashed component identity). However, the measures SQL output for that metric projects only the bare id column, so external consumers using the derived expression to evaluate against the measures output can't resolve the column.

PR #1931 introduced a grain_alias field on MetricComponent: for plain-column DISTINCT it was set to the bare column name, for complex DISTINCT (e.g., CASE/IF expressions) it stayed equal to the component name (hashed). The measures sql then routed plain-DISTINCT projections under grain_alias as an optimization, with the idea being that the bare column is already in the projection as a GROUP BY grain key, so there was no need to emit a redundant aliased copy.

To fix this, we add the register_limited_component helper function:

• For plain-column DISTINCT, it emits an extra <bare> AS <component.name> projection so that the combiner rewriters stay consistent.
• For complex DISTINCT, the behavior is unchanged.

This also adds backwards-compatibility for materializations created before the fix.

Test Plan

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

Deployment Plan

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