feat(webhook): validate/warn when AgentRuntime.spec.auth.outbound route has multiple audiences

Problem

The AgentRuntime CRD defines spec.auth.outbound[].audiences as an array (future-proofing), but AuthBridge's routing.Route struct only supports a single target_audience string.

Currently, the webhook silently takes the first audience and ignores any others:

// pod_mutator.go
if len(outboundRoute.Audiences) > 0 {
    route["target_audience"] = outboundRoute.Audiences[0]  // Only first is used
}

If a user specifies:

spec:
  auth:
    outbound:
      - destination:
          host: tool.svc.cluster.local
        audiences:
          - "spiffe://app1"
          - "spiffe://app2"  # ← SILENTLY IGNORED

The second audience is dropped without any feedback.

Proposed Solution

Add validation at webhook admission time:

if len(outboundRoute.Audiences) > 1 {
    mutatorLog.Info("Multiple audiences specified but only first will be used",
        "route", outboundRoute.Destination.Host,
        "audiences", outboundRoute.Audiences,
        "using", outboundRoute.Audiences[0])
    // Consider: add a warning event on the AgentRuntime CR
}

Alternative: Reject the AgentRuntime entirely if multiple audiences are specified (stricter, but clearer failure mode).

Context

• AuthBridge source: cortex/authbridge/authlib/routing/router.go - Route struct has Audience string
• CRD definition: operator/api/v1alpha1/agentruntime_types.go - has Audiences []string
• The array format was chosen for future-proofing, but creates a mismatch with current AuthBridge capabilities

Related

• Discovered during PR #517 review
• Potentially related to future multi-audience token exchange support in AuthBridge

/cc @alantech

Start with the webhook logic in pod_mutator.go, then read operator/api/v1alpha1/agentruntime_types.go and cortex/authbridge/authlib/routing/router.go to confirm the audiences mismatch. Resolve whether admission should warn or reject multiple audiences before implementing the chosen behavior. Done means multiple audiences receive explicit handling instead of being silently ignored.