fix(client): avoid nil-cache panic in Client.RemoveData

[sozercan]

Motivation

• handler.Cacher.GetCache() is permitted to return nil according to the interface contract, but Client.RemoveData previously dereferenced the returned cache unconditionally causing a nil-pointer panic when a handler returns nil and enabling an availability-impacting DoS during deletions.

Description

• Add a nil guard before calling Remove on a target handler cache in constraint/pkg/client/client.go so RemoveData checks cache != nil before invoking cache.Remove(relPath).
• Add a test helper nilCacheHandler that implements handler.Cacher and returns nil from GetCache() in constraint/pkg/client/client_test.go.
• Add TestClient_RemoveData_NilCache in constraint/pkg/client/client_test.go to verify RemoveData does not panic or return an error when a handler returns a nil cache.

Testing

• Ran go test ./pkg/client -run '^TestClient_RemoveData_NilCache$' -v inside the constraint/ module and the test passed.
• Ran go test ./pkg/client -run '^TestClient_RemoveData_Cache$' -v inside the constraint/ module and the test passed.
• Attempting go test ./constraint/pkg/client -run 'TestClient_RemoveData_(Cache|NilCache)$' from the repository root failed due to module layout (module is under constraint/), not due to the code change.

---

Codex Task

[Copilot]

Pull request overview

This PR hardens Client.RemoveData against nil-cache implementations of handler.Cacher, preventing a nil-pointer panic during deletions when a handler returns nil from GetCache() as permitted by the interface contract.

Changes:

• Add a nil guard around cache.Remove(relPath) in Client.RemoveData.
• Add a test-only nilCacheHandler that returns a nil cache.
• Add TestClient_RemoveData_NilCache to ensure RemoveData doesn’t panic and returns no error when a handler exposes a nil cache.

Show a summary per file

File	Description
constraint/pkg/client/client.go	Adds cache != nil guard before calling Remove in RemoveData to prevent nil dereference.
constraint/pkg/client/client_test.go	Introduces a nil-cache handler and a regression test validating RemoveData behavior with nil caches.

Copilot's findings

Tip

Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

• Files reviewed: 2/2 changed files
• Comments generated: 0