feat: Improved handling of unqualified SQL JOIN columns that are ambiguous

[alexander-beedie]

Closes #25748.

• The query in the linked Issue failing is expected, but the error message raised was not very helpful - this PR hugely improves the usefulness of the error message, and adds improves detection of ambiguous columns in JOIN clauses.

• In addition, reformulating the query in the linked Issue showed a different possible error; this was fixed by ensuring that the query each side of the UNION, or any other multi-frame set operation, uses execute_isolated (ref: fix: Solve multiple issues relating to arena mutation in SQL subqueries #25110) to prevent state leakage between the two queries (in this case, join aliases).

• Also, moved all the SQLVisitor helper implementations into their own location; am planning a big refactor of the polars-sql crate (which isn't that well structured at the moment), but will save that for its own PR later.

Example

import polars as pl
pl.sql("""
    WITH a AS (
        SELECT 0 AS k
    ), b AS (
        SELECT 0 AS k
    ), c AS (
        SELECT 0 AS k
    )
    SELECT k FROM a JOIN c on a.k = c.k
    UNION ALL
    SELECT k FROM b JOIN c on b.k = c.k
""").collect()

Before:

# ColumnNotFoundError: unable to find column "k:c"; valid columns: ["k"]

After:

# SQLInterfaceError: ambiguous reference to column "k" (use one of: a.k, c.k)

[alexander-beedie]

I see the new error-check actually found an ambiguous join column in the user guide; will fix that example when I get back home tonight 👍

Update: done.

[codecov]

Codecov Report

❌ Patch coverage is 93.60465% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.65%. Comparing base (20a1598) to head (6af6840).

Files with missing lines	Patch %	Lines
crates/polars-sql/src/sql_visitors.rs	93.26%	7 Missing ⚠️
crates/polars-sql/src/context.rs	94.11%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #25761    +/-   ##
========================================
  Coverage   80.64%   80.65%            
========================================
  Files        1765     1766     +1     
  Lines      243051   243163   +112     
  Branches     3044     3044            
========================================
+ Hits       196014   196113    +99     
- Misses      46255    46268    +13     
  Partials      782      782            

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Needs rebase

[alexander-beedie]

@nameexhaustion - rebasing now 👌
Looks like some clippy issues need addressing.