Fix source inference for source-less aggregate measures

[sahniaditya007]

Fixes #336

Summary

This PR adds source inference for source-less aggregate expressions in the semantic layer.

Previously, aggregate expressions without an explicit source.column reference (for example count(*), count(1), and sum(amount)) were rejected because no source could be inferred.

This change infers the source when the query already determines a single unambiguous source through its dimensions and/or other measures. Queries that remain ambiguous or have no inferable source continue to raise an error with improved guidance.

Changes

• Infer the source for source-less aggregate expressions when exactly one source can be determined.
• Preserve existing rejection behavior for ambiguous and zero-source queries.
• Improve error messages by:
  • listing candidate sources
  • suggesting count(source.pk) alternatives
• Add unit tests covering:
  • count(*)
  • count(1)
  • bare-column aggregates (sum(amount))
  • dimension-only inference
  • measure-only inference
  • mixed queries
  • dictionary measure definitions
  • ambiguous multi-source queries
  • zero-source queries

Testing

• Ran the semantic layer test suite.
• Added unit tests covering both successful inference and expected failure scenarios.

[vercel]

@sahniaditya007 is attempting to deploy a commit to the Kaelio Team on Vercel.

A member of the Team first needs to authorize it.