Rewrite createServicePolicy tests (#8666)

## Explanation

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The tests for `createServicePolicy` are too exhaustive and are very
painful to maintain. This commit rewrites the tests so that they focus
on covering all lines instead of all code paths.

Note that there are still some coverage issues with
`createServicePolicy` because Istanbul treats the re-exported Cockatiel
functions as untested. There doesn't seem to be a way around this short
of switching to v8. We may consider doing that in another commit, but
that is out of scope for now.

## References

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(Prerequisite for another PR that updates `createServicePolicy`)

## Checklist

- [x] I've updated the test suite for new or updated code as appropriate
- [ ] I've updated documentation (JSDoc, Markdown, etc.) for new or
updated code as appropriate
- [ ] I've communicated my changes to consumers by [updating changelogs
for packages I've
changed](https://github.com/MetaMask/core/tree/main/docs/processes/updating-changelogs.md)
- [ ] I've introduced [breaking
changes](https://github.com/MetaMask/core/tree/main/docs/processes/breaking-changes.md)
in this PR and have prepared draft pull requests for clients and
consumer packages to resolve them

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Test-only refactor with no production code changes; risk is limited to
possible gaps if the slimmer suite drops edge cases the old
combinatorial tests covered.
> 
> **Overview**
> **Replaces the ~3.7k-line `createServicePolicy` test suite with a
~1k-line version** aimed at line coverage and easier maintenance instead
of enumerating every combination of defaults and options.
> 
> The new structure groups cases by behavior (`execute` success/degraded
listeners, always-failing retries, circuit open vs closed, recovery
after `circuitBreakDuration`, custom `retryFilterPolicy` /
`isServiceFailure`, `httpStatus`, plus
`getRemainingCircuitOpenDuration`, `getCircuitState`, and `reset`).
Shared helpers **`createServicePolicyForTestingRetries`** (zero
`ConstantBackoff`, auto-advance timers on retry, optional
`breakAfterFirstExecution`) and **`createErroringService`** cut
duplicated mock/timer boilerplate.
> 
> **No changes to `create-service-policy` implementation** in this
diff—only tests and imports (`ConstantBackoff`, `ServicePolicy`).
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
1dd02ec. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: mcmire