Add quickstart smoke test to e2e suite

[HarshithaMS005]

What type of PR is this?
/kind test

What this PR does / why we need it:
The quickstart guide (site-src/guides/quickstart/) is the main way users try Agentic Networking, but nothing in CI exercised those manifests end-to-end. Existing e2e tests use custom testdata/ resources and a single /mcp route, so regressions in the quickstart YAML or policy wiring (for example #272) could break the quickstart without failing CI.
This PR adds TestQuickstartE2E to the existing e2e suite. It applies the real quickstart MCP server deployment and policy/e2e.yaml (Gateway, HTTPRoutes, XBackends, XAccessPolicies), then checks MCP calls through /local/mcp and /remote/mcp match the documented allow/deny behavior. It runs in the same Kind-based e2e job (pull-kube-agentic-networking-e2e) as the other controller tests, so manifest and policy mistakes are caught before users hit them in the quickstart.

Which issue(s) this PR fixes:
Fixes #281

Does this PR introduce a user-facing change?:

NONE

[k8s-ci-robot]

@HarshithaMS005: The label(s) kind/test cannot be applied, because the repository doesn't have them.

Details

In response to this:

What type of PR is this?
/kind test

What this PR does / why we need it:
The quickstart guide (site-src/guides/quickstart/) is the main way users try Agentic Networking, but nothing in CI exercised those manifests end-to-end. Existing e2e tests use custom testdata/ resources and a single /mcp route, so regressions in the quickstart YAML or policy wiring (for example #272) could break the quickstart without failing CI.
This PR adds TestQuickstartE2E to the existing e2e suite. It applies the real quickstart MCP server deployment and policy/e2e.yaml (Gateway, HTTPRoutes, XBackends, XAccessPolicies), then checks MCP calls through /local/mcp and /remote/mcp match the documented allow/deny behavior. It runs in the same Kind-based e2e job (pull-kube-agentic-networking-e2e) as the other controller tests, so manifest and policy mistakes are caught before users hit them in the quickstart.

Which issue(s) this PR fixes:
Fixes #281

Does this PR introduce a user-facing change?:

NONE

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: HarshithaMS005
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