[Bug]: Full frontend UT leaves RETURNING spool admission capped at one byte on main #29832

Is there an existing issue for the same bug?

• I have checked the existing issues.

Branch Name

main

Commit ID

590601d

Other Environment Information

• OS: macOS arm64
• Go: 1.27.1 (matching go.mod)
• Native tests: repository .agents/skills/mo-dev/scripts/mo-cgo-test wrapper with worktree-owned native artifacts
• GOMAXPROCS=2, GOFLAGS=-p=2; short temporary directory to avoid Unix-domain socket path limits

Actual Behavior

Full pkg/frontend UT fails on a clean checkout of the main commit above. Eight RETURNING spool tests reject small, otherwise valid records with an execution-resource cap of 1 byte:

TestReturningSpoolGenerationAndReplay:
internal error: execution resource admission rejected: requested=106 used=0 cap=1

TestDeferredReturningResponseStartsAfterPublish:
internal error: execution resource admission rejected: requested=114 used=0 cap=1

Other affected tests include TestDeferredReturningReplayOutputWaitStaysRootOwned, TestDeferredReturningPublishFailureDoesNotHideCommittedRows, TestDeferredReturningClientDisconnectCleansSpool, TestReturningSpoolRejectsTruncatedRecord, TestReturningSpoolRejectsCorruptMagic, and TestReturningSpoolReplayCancellationReleasesDecodeBudget.

The same failures occur after merging this main revision into #29816. They were then reproduced independently at the clean main commit, without any VARCHAR PR changes. This is separate from the original Ubuntu CI failure in TestPrivilegeCacheTracksRemoteCatalogChanges/grant_option_membership, which has not reproduced locally.

Expected Behavior

Full frontend UT should pass. Resource-limit tests and pooled session/process state should not leave subsequent RETURNING fixtures with an unintended 1-byte admission cap.

Steps to Reproduce

# Clean main checkout at 590601d62aa3c2fb6165d389ee710ed483c98da7;
# build native prerequisites in that worktree first.
mkdir -p /tmp/CLAUDE-frontend-ut
GOTOOLCHAIN=go1.27.1 GOMAXPROCS=2 GOFLAGS=-p=2 \
TMPDIR=/tmp/CLAUDE-frontend-ut \
.agents/skills/mo-dev/scripts/mo-cgo-test -v -count=1 -timeout=300s ./pkg/frontend

Additional information

Clean-main reproduction: package failure in 24.254s. Merged PR reproduction: package failure in 25.132s. The failing tests and error messages match. No assertions, sleeps, skips, or admission limits have been changed to hide the failures. Exact contamination/ownership root cause is still to be determined.