test: add unit coverage for ci writer helpers

[Ankitsinghsisodya]

test: add unit tests for BufferWriter in cmd/ci

Closes #3567

Matches the draft in pr1/issue.md.

What this PR does

Adds cmd/ci/writer_test.go — a new test file for cmd/ci/writer.go.

Before this change, the BufferWriter test double had zero direct test
coverage. This PR adds focused tests for the behaviours other cmd/ci tests
depend on: recording the path passed to Write, reporting existence based on
buffer content, and resetting the buffer on consecutive writes.

Changes

File	Change
cmd/ci/writer_test.go	New — 3 test functions for BufferWriter

Tests added

TestBufferWriter_WriteAndExist   // Exist is false before Write and true after Write
TestBufferWriter_Write_StoresPath // Write records the provided path
TestBufferWriter_Write_ResetsBuffer // consecutive writes replace content

How to verify

go test ./cmd/ci/... -v -run TestBufferWriter

Expected output: 3 tests PASS, ok knative.dev/func/cmd/ci.

Checklist

• Tests only — no production code changed
• make test passes
• make check passes
• No new dependencies introduced

[knative-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: Ankitsinghsisodya
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[knative-prow]

Hi @Ankitsinghsisodya. Thanks for your PR.

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[Copilot]

Pull request overview

Adds unit tests for the CI workflow writer helpers in cmd/ci/writer.go, addressing the lack of coverage noted in #3567.

Changes:

• Introduces cmd/ci/writer_test.go with unit tests covering fileWriter.Write, fileWriter.Exist, and the BufferWriter test double.
• Verifies parent directory creation, overwrite (truncate) behavior, and in-memory writer semantics (path recording + buffer reset).

---

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.24%. Comparing base (e92b074) to head (d5e6ba7).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3569      +/-   ##
==========================================
+ Coverage   55.52%   56.24%   +0.72%     
==========================================
  Files         180      180              
  Lines       20465    20465              
==========================================
+ Hits        11363    11511     +148     
+ Misses       7902     7755     -147     
+ Partials     1200     1199       -1     

Flag	Coverage Δ
e2e	36.10% <ø> (-4.07%)	⬇️
e2e go	32.55% <ø> (ø)
e2e node	28.36% <ø> (ø)
e2e python	32.91% <ø> (ø)
e2e quarkus	28.50% <ø> (+0.02%)	⬆️
e2e rust	27.92% <ø> (ø)
e2e springboot	26.34% <ø> (-0.03%)	⬇️
e2e typescript	28.47% <ø> (ø)
e2e-config-ci	18.14% <ø> (ø)
integration	17.40% <ø> (+<0.01%)	⬆️
unit macos-14	43.37% <ø> (ø)
unit macos-latest	43.37% <ø> (ø)
unit ubuntu-24.04-arm	43.57% <ø> (ø)
unit ubuntu-latest	44.25% <ø> (ø)
unit windows-latest	43.38% <ø> (-0.01%)	⬇️

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