fix(cmd): reject malformed subscribe filters

[Elvand-Lie]

Fixes #3941

Why

func subscribe --filter currently prints malformed filter values and continues, which can write a subscription without the intended filter.

What changed

• return an error for malformed --filter values
• preserve valid filter values containing =
• add regression tests for both paths

Testing

• gofmt -w cmd/subscribe.go cmd/subscribe_test.go
• git diff --check
• Added regression tests in cmd/subscribe_test.go

Note: focused go test ./cmd -run 'TestSubscribe(RejectsMalformedFilter|AllowsFilterValuesContainingEquals)' -count=1 -v was attempted locally on Windows but timed out before package output. CI should run the tests normally.

[knative-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[knative-prow]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 53.92%. Comparing base (89a927f) to head (6c466fd).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3942      +/-   ##
==========================================
+ Coverage   51.97%   53.92%   +1.95%     
==========================================
  Files         203      203              
  Lines       23807    23806       -1     
==========================================
+ Hits        12374    12838     +464     
+ Misses      10281     9739     -542     
- Partials     1152     1229      +77     

Flag	Coverage Δ
e2e	34.14% <42.85%> (-0.02%)	⬇️
e2e go	?
e2e node	?
e2e python	?
e2e quarkus	?
e2e rust	?
e2e springboot	?
e2e typescript	?
e2e-config-ci	?
integration	15.65% <0.00%> (?)
unit macos-14	43.34% <100.00%> (+0.01%)	⬆️
unit macos-latest	43.34% <100.00%> (+0.01%)	⬆️
unit ubuntu-24.04-arm	43.63% <100.00%> (+0.01%)	⬆️
unit ubuntu-latest	44.19% <100.00%> (+0.01%)	⬆️
unit windows-latest	43.38% <100.00%> (+0.01%)	⬆️

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