Add BBR end-to-end tests with multi-pool routing verification

[asaadbalum]

What type of PR is this?
/kind test

What this PR does / why we need it:

Adds a comprehensive Ginkgo/Gomega E2E test suite for the Body Based Router (BBR) that validates its core production behavior: request body parsing, model name extraction, base model lookup via ConfigMaps, header mutation (X-Gateway-Base-Model-Name with ClearRouteCache), and header-based routing to distinct backend clusters.

The test infrastructure deploys two model server simulators (Llama + DeepSeek) with LoRA adapters, an Envoy proxy with ext_proc filter routing to BBR, and header-based routing with x-bbr-routed-pool response headers for explicit pool verification.

Test cases:

1. Base model routing — verifies base models route to the correct pool via x-bbr-routed-pool header, covering both /completions and /chat/completions APIs.
2. LoRA adapter routing — verifies BBR resolves LoRA adapter names to the correct base model via ConfigMap lookup, routing to the correct pool.
3. Streaming routing — verifies streaming requests route correctly and SSE data: chunks arrive at the client.
4. BBR metrics — scrapes BBR's Prometheus endpoint and verifies bbr_success_total and bbr_info metrics.

Run with make test-e2e-bbr. Follows the same pattern as the existing EPP E2E tests.

Which issue(s) this PR fixes:

Relates to #2493

Does this PR introduce a user-facing change?:

NONE

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Note

Reviewer note — hack/test-e2e.sh unification

This PR modifies the existing hack/test-e2e.sh rather than adding a separate hack/test-e2e-bbr.sh script. The two scripts would have been ~95% identical (Kind setup, cluster detection, image loading), differing only in the image variable, make target, and test path.

The unified script uses a COMPONENT environment variable (epp by default, set by the Makefile targets), so make test-e2e continues to work identically with zero behavioral change for EPP.

Note: the existing test-e2e Makefile target still addresses EPP (not renamed to test-e2e-epp), since renaming it would affect the EPP README, conformance docs, and developer workflows — out of scope for this PR.

If reviewers prefer keeping separate scripts or renaming the targets, happy to adjust.

[k8s-ci-robot]

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