Add mocktest package: Silent TestingT for use inside Eventually

[lei-wego]

Summary

Adds a tiny new module `github.com/wego/pkg/mocktest` with a single export — `mocktest.Silent` — a `mock.TestingT` implementation that discards every call. Use it as the `TestingT` argument to testify mock assertion helpers (`AssertNumberOfCalls`, `AssertCalled`, `AssertNotCalled`, `AssertExpectations`) when polling inside `Eventually`.

Why

testify v1.11.0 changed `assert.Eventually` to evaluate the condition at t=0 before the first tick (stretchr/testify#1424). Mock assertion helpers all call `t.Errorf` on failure — so when called inside `Eventually` with the real test `T`, the initial poll (which runs before any async goroutine has had a chance to fire) permanently marks the test failed even when the condition becomes true on a later tick.

`mocktest.Silent` swallows the per-poll failure, letting the helpers be polled safely:

```go
assert.Eventually(t, func() bool {
return m.AssertNumberOfCalls(mocktest.Silent, "Foo", 1)
}, waitFor, tick)
```

Context

`wego/payments` currently works around this with a `replace github.com/stretchr/testify => v1.10.0` directive (added in wego/payments#1951) to keep ~106 legacy call sites green. Once this PR lands and is tagged, payments will adopt `mocktest.Silent`, refactor those sites, and drop the testify pin.

Tests

• `Silent` satisfies `mock.TestingT` (compile-time check).
• 4 runtime tests covering all four assertion helpers (`AssertNumberOfCalls`, `AssertCalled`, `AssertNotCalled`, `AssertExpectations`), verifying that:
  • mismatches don't mark the outer test failed
  • delayed async calls eventually satisfy the condition (the original bug)

Run: `cd mocktest && go test -v .` → 5/5 pass.

Test plan

• Local `go test -v .` clean
• Pre-push hook clean
• Wait for CI

After merge: tag as `mocktest/v0.1.0` so consumers can pin.