opt: drop provably-non-null COALESCE operands

[lohitkolluri]

Fixes #103596

Uses NotNullCols to trim leading COALESCE args in Project and Select. Example: SELECT COALESCE(i, j) FROM ij with i NOT NULL no longer keeps the coalesce in the plan.

Test plan: bazel test //pkg/sql/opt/norm:norm_test --test_filter=TestNormRules

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[lohitkolluri]

Friendly ping for review when you have time.

Fixes #103596 — implements SimplifyCoalesceProject / SimplifyCoalesceSelect using NotNullCols as discussed in the issue thread.

Test plan (run locally):

bazel test //pkg/sql/opt/norm:norm_test --test_filter=TestNormRules

@mgartner — you outlined the approach on the issue; would appreciate your eyes on the norm rules / testdata if you have bandwidth.

@mw5h — thanks if you're already on this via code owners.

Happy to iterate on any feedback. If full CI needs a maintainer trigger for community PRs, please let me know what to run.

[blathers-crl]

Detected infrastructure failure (matched: self-hosted runner lost communication with the server). Automatically rerunning failed jobs. (run link)

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