Remove testify from config/config_test.go

[renaudhartert-db]

Summary

Removes testify/assert and testify/require from config/config_test.go,
replacing them with standard library assertions and google/go-cmp.

Why

The project convention discourages testify in new and updated test code to
minimize third-party dependencies. config/config_test.go was one of the
files still importing testify. This PR brings it in line with the preferred
style already used in the second half of the file and in newer test files.

What changed

Interface changes

None.

Behavioral changes

None — pure test refactor.

Internal changes

• Consolidated four separate TestHostType_* functions into one table-driven
  TestConfig_HostType with four cases.
• Replaced assert.Equal/assert.NoError/require.NoError with
  if got != want { t.Errorf(...) } and if err != nil { t.Fatalf(...) }.
• Replaced assert.ErrorIs with errors.Is.
• Replaced assert.Panics with defer/recover.
• Replaced assert.True/assert.False with plain if checks.
• Used cmp.Diff for OAuthAuthorizationServer struct comparisons.
• Removed testify/assert and testify/require imports; added errors.

How is this tested?

All 21 tests in config/config_test.go pass locally via go test ./config/ -v.

NO_CHANGELOG=true

[simonfaltum]

Looks like a clean migration, thanks for doing this

[github-actions]

If integration tests don't run automatically, an authorized user can run them manually by following the instructions below:

Trigger:
go/deco-tests-run/sdk-go

Inputs:

• PR number: 1511
• Commit SHA: 77312d5d69f550f5133032305a902c89e28bf8cd

Checks will be approved automatically on success.