v3: use interface instead of `*testing.T` in `test_helpers`

[bigbes]

Refactor the test_helpers package to accept a custom T interface instead of concrete *testing.T. This makes test utilities reusable in examples, benchmarks, or custom test runners.

Types like MockDoer and MockResponse depend directly on *testing.T:

func NewMockDoer(t *testing.T, ...) *MockDoer

This tightly couples helpers to the testing package, preventing their use in:

• Example functions (ExampleXxx)
• Custom test frameworks
• Non-*testing.T contexts (e.g., internal validation tools)

Define a minimal interface that captures only the methods we actually use:

// T is a subset of *testing.T used for test helpers.
type T interface {
	Helper()
	Fatalf(format string, args ...interface{})
	Errorf(format string, args ...interface{})
}

Then update all functions in test_helpers to accept T instead of *testing.T.

Checklist

• Add the T interface in test_helpers (e.g., in test_helpers/types.go).
• Update all helper functions and structs (MockDoer, MockResponse, etc.) to use T.
• Update existing tests to still pass *testing.T (it satisfies T automatically).
• Modify CHANGELOG.md and MIGRATION.md accordingly