Add unit tests for `BufferWriter` in `cmd/ci/writer.go`

[Ankitsinghsisodya]

Issue: Add unit tests for BufferWriter in cmd/ci/writer.go

Summary

The BufferWriter test double in cmd/ci/writer.go currently has zero test
coverage on its Write and Exist methods.

Motivation

• BufferWriter is used as an in-memory WorkflowWriter fake in tests. If its
  behaviour changes silently, higher-level tests can start passing or failing
  for the wrong reason.
• Its behaviour is small but non-trivial: Write must record the path,
  replace existing buffer contents, and Exist must reflect whether any data
  has been written.

Proposed Change

Add a new file cmd/ci/writer_test.go (package ci_test) that covers:

Scenario	Method
Exist returns false before any write	BufferWriter.Exist
Exist returns true after a write	BufferWriter.Exist
Write stores the provided path	BufferWriter.Write
Consecutive writes reset the buffer instead of appending	BufferWriter.Write

Affected Files

• New: cmd/ci/writer_test.go

Labels

kind/test, good first issue

[Ankitsinghsisodya]

/assign

[Ankitsinghsisodya]

@matejvasek sir, what do you think about this?
First I thought about it, but when I look at the code more closely, it is just writing like test for test.
Should I close it?