fix: Fix append regression in 0.13.5

[priithaamer]

Attempt to fix #5494

[Copilot]

Pull Request Overview

This PR fixes a regression in the append functionality introduced in version 0.13.5. The fix addresses issue #5494 by improving the positional mapping logic for UNION operations in SQL generation.

• Adds bounds checking to prevent invalid array access when applying positional mappings
• Ensures positional mappings are only stored when complete (all columns matched)
• Adds defensive programming with logging for better debugging

Reviewed Changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 2 comments.

File	Description
prqlc/prqlc/src/sql/pq/positional_mapping.rs	Core fix adding bounds validation and completeness checks for positional mappings
prqlc/prqlc/tests/integration/queries/append_cte_complex.prql	New test case demonstrating complex append scenarios with CTEs
Multiple snapshot files	Updated test snapshots reflecting the regression fix and new test additions

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[max-sixty]

thanks @priithaamer . sorry for the regression.

let's merge? or any other feedback?

[max-sixty]

I did the same re reducing the integration tests for this — https://github.com/max-sixty/prql/tree/fix-5494-append-regression (or I can push to this MR with perms)

[eitsupi]

@priithaamer Could you take a look at this?

[priithaamer]

@eitsupi sorry for the delay. I've merged improvements from https://github.com/max-sixty/prql/tree/fix-5494-append-regression