feat(routing): make AI gateway route request timeout configurable

[Adam-D-Lewis]

Problem

The operator generates an AIGatewayRoute for each model's external
(llm.<baseDomain>) and internal (llm-internal.<baseDomain>) endpoint, but
it never set a request timeout. The Envoy AI Gateway therefore applies its
built-in 60s default (defaultRequestTimeout) to the HTTPRoute it
renders from each route. That is too short for many real LLM requests — long
or reasoning-heavy generations, large prompts, and cold model loads routinely
run past 60s and get cut off with a 504 / stream reset.

The route's own timeout is the only effective place to raise this:

• In Envoy Gateway an explicit HTTPRoute request timeout takes precedence
  over a BackendTrafficPolicy (the policy timeout is applied setIfNil), so
  a policy attachment cannot override the 60s.
• The route is controller-generated, so it cannot be patched from GitOps
  without fighting the operator's reconcile.

Change

Add a chart-wide Helm value defaults.routing.requestTimeout, plumbed
through:

values.yaml → LLM_ROUTE_REQUEST_TIMEOUT env (operator Deployment) →
config.OperatorConfig.RouteRequestTimeout → spec.rules[].timeouts.request
on both the external and internal AIGatewayRoutes.

It defaults to empty, which leaves the field unset and preserves the
gateway's existing 60s behavior on upgrade. Set it to a Gateway API duration
string to raise the timeout for all of a cluster's model routes:

defaults:
  routing:
    requestTimeout: "600s"

Testing

• go test ./internal/config/... ./internal/controller/reconcilers/... — new
  cases assert both routes carry timeouts.request when set and omit the
  block when empty; the config loader parses the new env var.
• go build ./..., gofmt, go vet clean.
• helm lint + helm template verified: no timeout rendered by default, the
  configured duration when set.

Follow-up

A per-model override on LLMModel.spec (so individual models can set their
own timeout, overriding this chart-wide default) is left for a separate PR.