fix: planner should error if join columns are missing

[stuarttimwhite]

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Rationale for this change

What changes are included in this PR?

Are these changes tested?

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.77%. Comparing base (3c43ab1) to head (7a54647).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/proof-of-sql-planner/src/plan.rs	80.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1104   +/-   ##
=======================================
  Coverage   96.77%   96.77%           
=======================================
  Files         302      302           
  Lines       59220    59223    +3     
=======================================
+ Hits        57310    57313    +3     
  Misses       1910     1910           

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[Copilot]

Pull request overview

This PR fixes a bug in the join planner where missing join columns were silently ignored instead of raising an error. Previously, the code used filter_map which would skip over columns that couldn't be found in the table schemas, potentially leading to incorrect join operations. The fix changes to map and explicitly converts None values from get_index_of() to PlannerError::ColumnNotFound.

Key Changes

• Changed from filter_map to map in join column processing to ensure all columns are validated
• Added explicit error handling using ok_or(PlannerError::ColumnNotFound)? when join columns are not found in table schemas
• This ensures the planner fails fast with a clear error message instead of silently continuing with incomplete join information

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[Copilot]

The new error handling for missing join columns (ColumnNotFound errors on lines 334 and 337) lacks test coverage. Consider adding a test case that verifies a ColumnNotFound error is raised when join columns are not present in the left or right table schemas.

Example test structure:

#[test]
fn we_cannot_convert_join_with_missing_column() {
    // Create tables where join column doesn't exist in one or both tables
    // Attempt to join on the missing column
    // Assert that result matches Err(PlannerError::ColumnNotFound)
}