fix: Lineage traces through CTEs to source tables

[nightscape]

Fixes lineage tracking for CTEs (Common Table Expressions) so they properly trace back to their underlying source tables instead of being treated as opaque tables.

Problem

When using a CTE like:

let employees_usa = (from employees | filter country == "USA")
from employees_usa | select {name, salary}

The lineage would incorrectly show:

inputs:
- name: employees_usa
  table: [employees_usa]  # Wrong - CTE treated as opaque

Solution

Modified lineage_of_table_decl in the resolver to check if a table declaration is a CTE (TableExpr::RelationVar) and, if so, trace back to the underlying source tables.

The lineage now correctly shows:

inputs:
- name: employees_usa
  table: [default_db, employees]  # Correct - traces to source

For CTEs with multiple inputs (UNIONs, JOINs), all source tables are included:

inputs:
- name: combined
  table: [default_db, employees]
- name: combined
  table: [default_db, contractors]

Changes

• prqlc/prqlc/src/semantic/resolver/inference.rs: Modified lineage_of_table_decl to trace CTE lineage to underlying source tables
• prqlc/prqlc/src/semantic/resolver/mod.rs: Added unit tests for simple CTEs and CTEs with UNIONs
• Updated snapshot: integration__queries__debug_lineage__genre_counts.snap reflects the corrected lineage behavior

Testing

• Added test_cte_lineage_traces_to_source_table - verifies simple CTE lineage
• Added test_cte_lineage_with_union_traces_to_all_source_tables - verifies UNION CTE lineage
• All existing tests pass

[max-sixty]

I tried to simplify a bit, with Claude's help — feel free to revert if you preferred the former though

[nightscape]

@max-sixty
I'm fine with your version, thanks for the test 👍
Good to merge?