feat: add conflict detection for gateway proxy

[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:

add conflict detection for GatewayProxy resources that share the same admin credentials while targeting the same service or overlapping control-plane endpoints.

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 implements conflict detection for GatewayProxy resources to prevent multiple proxies from sharing the same admin credentials while targeting the same service or overlapping control-plane endpoints. This helps avoid configuration issues that could arise from credential conflicts between gateway proxy instances.

Key Changes:

• Added validation logic to detect and reject conflicting GatewayProxy configurations during create and update operations
• Enhanced the webhook validator with comprehensive conflict detection for both service-based and endpoint-based gateway proxy configurations
• Added comprehensive test coverage for various conflict scenarios including inline admin keys and secret-based admin keys

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
internal/webhook/v1/gatewayproxy_webhook.go	Implements core conflict detection logic with validation methods and configuration comparison utilities
internal/webhook/v1/gatewayproxy_webhook_test.go	Adds comprehensive unit tests covering service conflicts, endpoint overlaps, and admin key sharing scenarios
test/e2e/webhook/gatewayproxy.go	Adds end-to-end tests for webhook conflict detection with real resource creation and validation
internal/controller/indexer/tlsroute.go	Minor import reordering for consistency

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