Remove testify from SDK modules

[zeevdr]

Description

Remove testify dependency from all SDK modules (configclient, adminclient, configwatcher). Replace with stdlib testing + hand-written mocks.

Testify is only used in test files but appears as a direct dependency in go.mod, which can cause version conflicts in consumers' go.sum.

Acceptance criteria

• Remove github.com/stretchr/testify from go.mod in configclient, adminclient, configwatcher
• Replace assert/require with stdlib equivalents
• Replace testify/mock with hand-written mock structs
• All tests pass
• No testify imports remain in SDK modules

Notes

• sdk/tools has no testify dependency (already clean)
• cmd/decree can keep testify (it's an end-user binary, not a library)
• Phase 1 of SDK dependency reduction (see SDK transport interface: decouple from gRPC #160 for transport interface)