adapter: don't use max_query_result_size in RTW plans

[maddyblue]

Avoid max_query_result_size in the read portion of read-then-write plans. Previously that session var would limit the size of the updates to its setting. This would cause, for example, a DELETE issued from the console to fail if there were more than 1MB of rows in a table.

Fixes MaterializeInc/database-issues#8099

Motivation

• This PR fixes a recognized bug.

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).
• This PR includes the following user-facing behavior changes:
  • n/a

[maddyblue]

Not convinced this is the best way to do it (there's a bunch of plumbing down to the last step), but this is the common place where RTW and user peeks happen, so kinda makes sense.