Don't panic when the same parameter is coerced twice

[ggevay]

Fixes a panic when plan_coerce is called twice on a parameter. This typically happens when there is a function call whose more than one argument involves the same prepared statement.

Motivation

• This PR fixes a recognized bug: https://github.com/MaterializeInc/database-issues/issues/9268

Tips for reviewer

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.