What this fixes

A BackendTrafficPolicy that resolves to zero targets currently leaves its status stale. This PR makes the translator report such policies explicitly. It addresses two separate issues with the same symptom but different root causes:

• Fixes BackendTrafficPolicy status does not report missing targetRef and leaves stale observedGeneration #8926 — targetRef/targetRefs pointing at a nonexistent object. The policy is added to the result set, but processBackendTrafficPolicyForRoute/processBackendTrafficPolicyForGateway return early once the target object cannot be resolved (targetedRoute == nil / targetedGateway == nil), so no ancestor status is ever written. The result: Accepted=True persists with a stale observedGeneration and no negative condition.
• Fixes BackendTrafficPolicy status becomes stale when targetSelectors change from matching routes to matching no routes #8927 — targetSelectors that match no objects. The policy is never added to the result set at all (selector resolution returns nothing), so its status is never reconciled and stays stale.

The fix

After all target-processing loops in ProcessBackendTrafficPolicies, any policy that resolved to zero ancestors is now reported with:

• Accepted = False
• reason TargetNotFound
• the current metadata.generation as observedGeneration

Policies that attached to at least one target are left untouched. Selector-only policies that matched nothing are added to the result set so their status is published.

Follows an existing pattern in the repo

This change reuses the status-reporting building blocks already used across the codebase rather than introducing new ones:

• The gwapiv1.PolicyReasonTargetNotFound reason is already used in the repo for the "section within an existing target not found" case (e.g. validate.go and resolveClientTrafficPolicyTargetRef in clienttrafficpolicy.go). This PR extends that same reason to the "target object entirely missing / zero targets matched" case.
• Conditions are set with the existing status.SetConditionForPolicyAncestor helper, the same helper already used throughout ProcessBackendTrafficPolicies.

Design note — ancestor reference for unattached policies

Per the Gateway API, each PolicyAncestorStatus entry must carry an ancestorRef. When a BackendTrafficPolicy resolves to zero targets there is no resolved Gateway to use as the ancestor, so this PR chooses one deterministically:

• If the policy uses an explicit targetRef/targetRefs, the (unresolved) target is used as the ancestorRef, so the reported status points directly at the missing object.
• If the policy uses only targetSelectors (which reference no named object), the policy references itself as the ancestorRef.

I'd appreciate maintainer confirmation that this ancestor convention is acceptable, since the Gateway API doesn't prescribe one for the "attached to nothing" case.

Scope

This PR is intentionally scoped to BackendTrafficPolicy to keep it focused on the two filed issues. The same early-return / never-added gap exists in the sibling policy types (SecurityPolicy, ClientTrafficPolicy, and others). If the maintainers are happy with the approach and the ancestor convention above, I'm glad to generalize it across the other policy types in a follow-up PR.

Testing

• New golden fixtures reproducing each case:
  • backendtrafficpolicy-targetref-not-found (BackendTrafficPolicy status does not report missing targetRef and leaves stale observedGeneration #8926)
  • backendtrafficpolicy-selector-no-match (BackendTrafficPolicy status becomes stale when targetSelectors change from matching routes to matching no routes #8927)
• Existing goldens updated to reflect corrected behavior:
  • backendtrafficpolicy-status-conditions — four not-found policies that previously produced ancestors: null now report TargetNotFound.
  • policy-cross-namespace-targetselector-missing-referencegrant-{gateway,route} — the policy previously vanished from output entirely and is now reported.
• go test ./internal/gatewayapi/... passes; go vet and gofmt are clean.

Release note

Added release-notes/current/bug_fixes/8926-btp-status-target-not-found.md.

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