Add CEL validation for unsupported BTP fields on TCPRoute/UDPRoute targets

## Summary

Some `BackendTrafficPolicySpec` fields are silently ignored when the policy targets a `TCPRoute` or `UDPRoute`. For example, HTTP-specific fields like `rateLimit`, `faultInjection`, `compression`, `responseOverride`, `httpUpgrade`, `requestBuffer`, and `telemetry` have no effect on TCP/UDP routes but are not rejected at admission time.

This is inconsistent with the CEL validation added for Backend targets in #8890, which explicitly rejects unsupported fields via CEL rules.

## Proposal

Add CEL validation rules to reject fields that are not applicable when the `targetRef` kind is `TCPRoute` or `UDPRoute`, similar to how Backend-targeted BTPs reject RDS-only fields.

## Context

Raised in [PR #8890 review](https://github.com/envoyproxy/gateway/pull/8890#discussion_r2128652942) by @zhaohuabing as a non-blocking nit for consistency.