fix(cmd): reject malformed subscribe filters

[axhatggg]

Fixes #3941

Why

func subscribe --filter accepted malformed values, printed "Invalid pair:",
and continued — then wrote func.yaml anyway and exited 0. A user who typos a
filter in a script gets a silent misconfiguration: the function subscribes more
broadly than intended, and it only surfaces much later.

Separately, strings.Split was used where SplitN is needed, so legitimate
filter values containing "=" (e.g. foo=bar=baz) were also discarded.

What changed

• use strings.SplitN(filter, "=", 2) so filter values containing "=" are preserved
• extractFilterMap returns an error for a missing "=" or an empty key
• the error propagates through updateOrAddSubscription to runSubscribe, so the
  command fails before f.Write() rather than after

Testing

• TestExtractFilterMap — table-driven, 8 cases: valid filters, values containing
  "=", empty values, missing "=", and empty keys
• TestSubscribeRejectsMalformedFilter — asserts cmd.Execute() returns an error
  and that func.yaml is left untouched
• TestSubscribeAllowsFilterValueContainingEquals — asserts foo=bar=baz round-trips
• Both end-to-end tests were confirmed failing before the fix was applied
• make test and make check pass locally (Go 1.26.7, Linux)

[linux-foundation-easycla]

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• ✅ login: axhatggg / name: axhatggg (6cbfb89)

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[knative-prow]

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