Heatstack controller handles secret not found error

This MR's purpose is to add a case when the secret is deleted too early and returns an unauthorized error. A unit test is also added.

There are some changes regarding golangci-lint that was updated from

golangci/golangci-lint:v1.64.8-alpine --> golangci/golangci-lint:v2.11.4-alpine

and linter changes inside the code.

Linked to issue: #27 (closed)

Tested in MR: sylva-projects/sylva-core!7772 (closed)

Closes #27 (closed)