refactor: SetRouteConditionResolvedRefs function

[ronething]

Type of change:

• Bugfix
• New feature provided
• Improve performance
• Backport patches
• Documentation
• Refactor
• Chore
• CI/CD or Tests

What this PR does / why we need it:

Pre-submission checklist:

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?
• Is this PR backward compatible? If it is not backward compatible, please discuss on the mailing list first

[Copilot]

Pull Request Overview

This PR refactors how the resolved references condition is set for routes by replacing separate status and message parameters with an error object and by introducing a dedicated error type for backend not found situations.

• Updates the SetRouteConditionResolvedRefs function in utils.go to accept an error instead of a boolean status and message.
• Modifies httproute_controller.go to use error objects for backend reference resolution and to correctly prioritize invalid kind errors.
• Adds a new BackendNotFoundError type with helper functions for error handling.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
internal/controller/utils.go	Refactors the condition-setting function and adds a new error type for backend not found.
internal/controller/httproute_controller.go	Adjusts error handling logic for backend refs and updates status reporting accordingly.

[github-actions]

conformance test report

apiVersion: gateway.networking.k8s.io/v1
date: "2025-05-16T10:35:29Z"
gatewayAPIChannel: standard
gatewayAPIVersion: v1.2.0
implementation:
  contact: null
  organization: APISIX
  project: apisix-ingress-controller
  url: https://github.com/apache/apisix-ingress-controller.git
  version: v2.0.0
kind: ConformanceReport
mode: default
profiles:
- core:
    result: partial
    skippedTests:
    - GatewaySecretInvalidReferenceGrant
    - GatewaySecretMissingReferenceGrant
    - HTTPRouteHTTPSListener
    - HTTPRouteInvalidCrossNamespaceBackendRef
    - HTTPRouteInvalidReferenceGrant
    - HTTPRoutePartiallyInvalidViaInvalidReferenceGrant
    - HTTPRouteReferenceGrant
    statistics:
      Failed: 0
      Passed: 26
      Skipped: 7
  name: GATEWAY-HTTP
  summary: Core tests partially succeeded with 7 test skips.