testifylint: enable the go-require linter and fix all findings

[arthurschreiber]

Description

This enables testifylint's go-require checker and fixes every finding across the codebase.

go-require flags require.* calls made from a non-test goroutine. require calls t.FailNow() → runtime.Goexit(), which only terminates the test correctly on the goroutine running the test function. Off that goroutine it kills just the spawned goroutine, so the test can hang on a channel that never closes, or report a false pass — a real correctness problem, not a style nit.

The PR is structured so each piece is easy to review:

• Enable the checker in .golangci.yml with only go-require turned on (disable-all: true + enable: [go-require]). The other testifylint checkers are being adopted separately.
• Unit-test findings (topo conformance helpers, conn_test, tm_init_test, vstreamer_test, workflow utils_test): require → assert inside the spawned goroutines, with explicit return/continue so the abort behaviour require's Goexit provided is preserved. Shared helpers used from both the test goroutine and a child goroutine (createSocketPair, grantAllPrivilegesToUser, startFullyThrottledStream) are fixed at the goroutine call site so their main-goroutine callers stay fail-fast.
• execVtgateQuery / insertLargeTransactionForChunkTesting now return an error instead of asserting internally (the *testing.T parameter is dropped); ~108 call sites updated to require.NoError on the test goroutine and assert.NoError in the inserter goroutines. This avoids a parallel error-returning "twin" helper.
• endtoend findings, including making waitForWorkflowState and performVDiff2Action return errors so they are goroutine-safe (again, instead of carrying a twin). performVDiff2Action keeps waiting for the workflow internally via the now-require-free waitForWorkflowState.

go vet passes across every touched package, and golangci-lint run --max-same-issues 0 --max-issues-per-linter 0 ./go/... reports 0 go-require findings. The unit-test fixes were verified locally with go test -race; the endtoend changes rely on CI for behavioural validation.

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

None — this only touches test code and the linter config.

AI Disclosure

This PR was written primarily by Claude Code — I provided direction and reviewed the changes.

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[vitess-bot]

Review Checklist

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• Ensure that the Pull Request has a descriptive title.
• Ensure there is a link to an issue (except for internal cleanup and flaky test fixes), new features should have an RFC that documents use cases and test cases.

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• There should be a comment at the top of each new or modified test to explain what the test does.

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.33%. Comparing base (70c7a72) to head (7a3018f).
⚠️ Report is 352 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/topo/test/shard.go	0.00%	7 Missing ⚠️
go/vt/topo/test/trylock.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main   #20397       +/-   ##
===========================================
- Coverage   69.67%   69.33%    -0.34%     
===========================================
  Files        1614      279     -1335     
  Lines      216793    46990   -169803     
===========================================
- Hits       151044    32580   -118464     
+ Misses      65749    14410    -51339     

Flag	Coverage Δ
partial	69.33% <0.00%> (?)

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