fix: stale HTTPRoute/GRPCRoute/TLSRoute/TCPRoute/UDPRoute status after gateway-only spec changes

[yuehaii]

What type of PR is this?
fix: stale HTTPRoute/GRPCRoute/TLSRoute/TCPRoute/UDPRoute/ status after gateway-only spec changes

Issue description
When a Gateway resource is updated without touching any Route resource (e.g. a listener hostname or allowedRoutes change), the Route's status.parents[*].conditions become stale, the observedGeneration field continues pointing at the old gateway generation instead of the new one.

Root cause
The bug only manifests as False→False (same outcome, only ObservedGeneration should advance). If the gateway update trigger a new route reconcile, GetGeneration() hasn't changed because the route was not modified. So the newly computed condition is byte-for-byte identical to what's already in the store and never trigger the status update.

Fix
Compute observedGeneration as the max of route and gateway generation at the top of each parentRef loop iteration, and use it only for gateway-driven conditions.

Verification

1. unit test

go test ./internal/gatewayapi/runner/... -v -run "TestHTTPRouteStatusStaleAfterAllowedRoutesChange" -count=1 2>&1 | tail -15
=== RUN TestHTTPRouteStatusStaleAfterAllowedRoutesChange
--- PASS: TestHTTPRouteStatusStaleAfterAllowedRoutesChange (0.15s)
PASS
1.777439888886663e+09 info gateway-api runner/runner.go:557 shutting down {"runner": "gateway-api"}
ok github.com/envoyproxy/gateway/internal/gatewayapi/runner 1.612s

go test ./internal/gatewayapi/runner/... -v -run TestHTTPRouteStatusStaleAfterListenerHostnameChange -count=1 2>&1 | tail -50
=== RUN TestHTTPRouteStatusStaleAfterListenerHostnameChange
--- PASS: TestHTTPRouteStatusStaleAfterListenerHostnameChange (0.15s)
PASS
1.7774398311383302e+09 info gateway-api runner/runner.go:557 shutting down {"runner": "gateway-api"}
ok github.com/envoyproxy/gateway/internal/gatewayapi/runner 1.663s

2. integration test

Step 1 — Bug Reproduction (unpatched controller)

Setup: Gateway listener on other.example.com, HTTPRoute hostname test.example.com (no match).

Step	Gateway gen	Accepted condition	observedGeneration
Create gateway + route	1	False / NoMatchingListenerHostname	1
Add matching listener (gateway-only edit)	2	False / NoMatchingListenerHostname ❌ (stale — should be True)	still 1 ❌

---

Step 2 — Fix Verification (patched controller)

Step	Gateway gen	Accepted condition	observedGeneration
Reset: only other.example.com listener	3	False / NoMatchingListenerHostname	3
Add another.example.com (same False outcome, only gen changes)	4	False / NoMatchingListenerHostname	4 ✅
Add matching test.example.com listener	5	True / Accepted	5 ✅

Which issue(s) this PR fixes:
Fixes #8861

Release Notes: Yes

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