event mute and event cancel report success on events they cannot affect

[leggetter]

Summary

gateway event mute and gateway event cancel print a success message and exit 0 on an event where they cannot have had any effect, and reading the event back shows nothing changed.

Reproduce

On an event in a terminal FAILED state:

hookdeck gateway event mute <id>      → Event <id> muted.      exit 0
hookdeck gateway event cancel <id>    → Event <id> cancelled.  exit 0
hookdeck gateway event get <id> --output json
→ no field changed

For contrast, gateway event retry on the same event genuinely worked — attempts went 1 → 2.

Assessment

This is plausibly a legitimate server-side no-op: a terminal event cannot be cancelled or muted. The defect is that the CLI asserts it happened. Event <id> muted. is a statement about state that is not true.

Expected

Either report that the event was not in a state where the action applies, or report what the API actually did. A confirmation message should not be printed unconditionally on a 2xx that changed nothing.

Scope

Pre-existing, not a regression. Verified identical on fbb924a and the merged branch.

Why it matters

Same shape as several defects fixed in #392: the command reports success while nothing happened. Someone muting a noisy event believes it is muted.

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Filed by Claude on Phil's behalf, from release-candidate regression testing.