Add tests for named HTTPRouteRule sectionName targeting

## Summary

Gateway API introduced explicit naming for `HTTPRouteRule` entries ([kubernetes-sigs/gateway-api#995](https://github.com/kubernetes-sigs/gateway-api/issues/995)), allowing users to set a `name` field on individual rules in `spec.rules[].name` instead of relying on auto-generated names like `rule-1`, `rule-2`. Kuadrant's policy-machinery added support for this in [Kuadrant/policy-machinery#63](https://github.com/Kuadrant/policy-machinery/pull/63).

The testsuite currently targets HTTPRoute rules via auto-generated sectionName values (e.g. `"rule-1"`). We need tests that verify policies can target explicitly named HTTPRouteRules.

## Suggested Code Changes

### 1. `HTTPRoute.add_rule()` and `add_backend()` — add `name` parameter

In `testsuite/gateway/gateway_api/route.py`, both `add_rule()` and `add_backend()` build rule dicts without a `name` field. Add an optional `name` parameter so tests can create named rules:

```python
@modify
def add_backend(self, backend: "Backend", prefix="/", name: str = None):
rule = {"backendRefs": [backend.reference], "matches": [{"path": {"value": prefix, "type": "PathPrefix"}}]}
if name:
rule["name"] = name
self.model.spec.rules.append(rule)
```

Same change for `add_rule()`.

### 2. New test coverage

Add tests (e.g. under `testsuite/tests/singlecluster/limitador/section/` and `testsuite/tests/singlecluster/gateway/authpolicy/`) that:

- Create an HTTPRoute with explicitly named rules (e.g. `name="get-rule"`, `name="anything-rule"`)
- Attach a policy targeting one named rule via `section_name="get-rule"`
- Verify the policy applies only to the targeted rule and not the other

This should mirror the existing pattern in `test_route_rule.py` and `test_authpolicy_section_targeting_http_route.py`, but use explicit rule names instead of auto-generated ones.

Start in testsuite/gateway/gateway_api/route.py and compare test_route_rule.py with test_authpolicy_section_targeting_http_route.py. Add named HTTPRouteRule setup and tests in the indicated section directories, then verify a policy targeting section_name="get-rule" applies only to that rule and not the other named rule.

Written by the indexing model from the issue text.