conformance: `GatewayFrontendClientCertificateValidationInsecureFallback` feature

## Problem Statement

The upstream feature `GatewayFrontendClientCertificateValidationInsecureFallback` is part of Gateway extended conformance and is checked by the test in https://github.com/kubernetes-sigs/gateway-api/blob/main/conformance/tests/gateway-with-clientcertificate-validation-insecure-fallback.go.

This feature verifies support for the `AllowInsecureFallback` frontend client certificate validation mode.

Tracked as part of #1446.

## Proposed Solution

Investigate whether Kong Operator can support this feature in Gateway mode, implement any missing behavior if needed, and enable the feature in Gateway API conformance.

## Additional Information

- Feature: `SupportGatewayFrontendClientCertificateValidationInsecureFallback` (extended)
- Gateway API version introducing the feature: `v1.5.0`
- This feature depends on `SupportGatewayFrontendClientCertificateValidation`.

## Acceptance Criteria

- [ ] Root cause or compatibility status is identified and documented
- [ ] If compatible, the implementation is updated as needed
- [ ] `features.SupportGatewayFrontendClientCertificateValidationInsecureFallback` is enabled in `pkg/gatewayapi/supportedfeatures.go`
- [ ] The relevant Gateway conformance test passes in CI

Start with pkg/gatewayapi/supportedfeatures.go and compare the operator's current support for frontend client certificate validation with the upstream test in conformance/tests/gateway-with-clientcertificate-validation-insecure-fallback.go. Determine the compatibility status, document the root cause or findings, update behavior if required, enable the feature, and verify the relevant conformance test passes in CI.

Written by the indexing model from the issue text.