feat: implement graceful-teardown checker

[mmorel-35]

Implements the graceful-teardown checker proposed in CONTRIBUTING.md. Using require in t.Cleanup callbacks or suite teardown methods calls runtime.Goexit(), which aborts the goroutine mid-cleanup and can leave resources unreleased.

Checker

Detects require.* calls (including s.Require().Method()) inside:

• t.Cleanup(func() { ... })
• Suite teardown methods: TearDownTest, TearDownSuite, AfterTest, HandleStats, TearDownSubTest

// ❌ detected
func (s *Suite) TearDownTest() {
    s.Require().NoError(s.producer.Close()) // goroutine exits here; consumer never closed
    s.Require().NoError(s.consumer.Close())
}

t.Cleanup(func() {
    require.NoError(t, db.Close())
})

// ✅ fixed
func (s *Suite) TearDownTest() {
    s.Assert().NoError(s.producer.Close())
    s.Assert().NoError(s.consumer.Close())
}

t.Cleanup(func() {
    assert.NoError(t, db.Close())
})

Autofix

Two fix strategies:

Pattern	Fix
require.X(t, ...)	Replace require qualifier with assert; add assert import if absent (via addImportFix)
s.Require().X(...)	Replace Require() with Assert() in the method chain

The addImportFix helper (ported from the http-const branch) handles existing imports, aliases, name conflicts with top-level declarations, and insertion into the correct import group.

Configuration

• Enabled by default: false
• Autofix: true

Test coverage

• Generated test + golden file for the main cases (both packages already imported)
• Manual graceful-teardown-no-import test verifying the autofix correctly inserts the assert import when the file only imports require

Closes #142