[repr types] Fallback to repr type checks before failing

[mgree]

Recent (reverted) changes to ColumnKnowledge and enable_cast_elimination tickled some assertions we have in our code that assume we will have SQL typechecking at the MIR level. This PR relaxes those checks to try for repr typing as a fallback, using tracing::error! to notify when this fallback occurs (so we can track it in Sentry).

There were two places these checks were happening:

• In SqlColumnType::union, which was frequently .unwrap()ed.
• In an assertion for normalize_lets.

Scanning through 1055 assertions in the expr, repr, sql, and transform crates found no others, nor were the 42 uses of base_eq across the codebase suspect.

Motivation

• This PR fixes a recognized bug.
  https://github.com/MaterializeInc/database-issues/issues/10045
  https://github.com/MaterializeInc/database-issues/issues/10046
  https://github.com/MaterializeInc/database-issues/issues/10052

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.

[ggevay]

For the test fails:

• 💡 SQL logic tests (4 replicas) 4: You could put your file into tests_without_views_and_replica, so that it's not run with multiple replicas.
• 🏎️ testdrive 4 replicas: Just a rare flake, I've hit retry.
• Orchestratord + upgrade, combine props: common flake

[ggevay]

Here and the below two changed calls, why not try_union? I think the important thing for these calls (see code comment above) is to match typ's handling of If, which does a proper repr union, not a sql_union.

[mgree]

Good catch, thank you! I found two more instances in there. I thought I got all these... I must have not saved properly or something.

I'll trigger another nightly run including the relevant tests (sqllogictest with and without multiple replicas, testdrives) and then merge.