Use %w for error wrapping in controller.cancelDeployment

[mvanhorn]

Summary

cancelDeployment now wraps the error returned by ReportDeploymentCompleted with %w instead of %v, so callers can use errors.Is/errors.As to inspect the underlying cause.

Why this matters

The cancellation path formatted the reporting error with %v, which flattens it to a string and breaks error-chain inspection for any caller that wants to branch on the wrapped error. Switching to %w preserves the chain while keeping the existing message format. Reported in #6932.

Testing

Added a regression test that drives cancelDeployment with a fake API client returning a sentinel error and asserts errors.Is matches the wrapped error, so the test fails if the wrapping regresses to %v. The controller package tests pass.

Closes #6932