fix(x402): strip healthPath from buyer endpoint URL construction

[bussyjd]

Summary

buy.py's _normalize_endpoint only stripped trailing /v1/chat/completions and /chat/completions suffixes. When a seller declared a non-trivial upstream.healthPath (e.g. /api/tags for Ollama) and that segment leaked into the --endpoint URL, the spec builder happily re-appended /v1/chat/completions on top, producing dead URLs like http://traefik.../services/demo-hello/api/tags/v1/chat/completions.

Repro

Seller:

obol sell http demo-hello --upstream ollama --port 11434 --namespace llm \
  --health-path /api/tags --per-request 0.001 --chain base-sepolia --wallet 0x...

Buyer:

buy.py buy demo-hello --endpoint <URL> --model demo --count 1

Before fix: PurchaseRequest.spec.endpoint = http://traefik.../services/demo-hello/api/tags/v1/chat/completions. LiteLLM paid/demo POSTs through the x402-buyer sidecar to that URL → 404 from Traefik.

Root cause

buy.py (skill layer). The seller-side surface was clean:

• internal/serviceoffercontroller/render.go and controller.go only ever emit offer.EffectivePath() (= /services/<name>) for storefront / /skill.md / registration / status.Endpoint. EffectiveHealthPath() is consumed exactly once, by the controller's upstream health probe at controller.go:589.
• internal/x402/serviceoffer_source.go sets RouteRule.UpstreamURL to http://<svc>.<ns>.svc.cluster.local:<port> (no healthPath) and StripPrefix to offer.EffectivePath().
• internal/x402/forwardauth.go::buildResourceURL echoes the incoming request URI back as the 402 resource.url, but the seller never advertises a URL with healthPath in accepts[].

So the bug was confined to the buyer: it accepted any user-supplied URL containing /services/<name>/<extra>... and didn't sanitise the <extra> tail before appending /v1/chat/completions.

Fix

_normalize_endpoint now also detects /services/<segment>/... paths and truncates to /services/<segment> before appending the LLM suffix. External (non-/services) sellers are untouched.

Test plan

• Added tests/test_buy_normalize_endpoint.py (13 cases) covering:
  • existing trailing-slash / /v1/chat/completions / /chat/completions strip behaviour preserved
  • canonical /services/<name> URL unchanged
  • healthPath segments /api/tags, /health, nested paths, and /api/tags/v1/chat/completions all collapse to /services/<name>
  • assembled <normalized>/v1/chat/completions matches canonical buyer URL
  • non-/services URLs left intact (external x402 sellers)
• go build ./... clean
• go test ./internal/x402/... ./internal/serviceoffercontroller/... green
• go test ./... green except one pre-existing failure (TestWarnIfNoChatModel_EmitsWarnWhenNoModels in internal/stack — unrelated, confirmed against main)
• Existing Python tests in tests/test_buy_autorefill.py still pass (4 pre-existing socket failures from offline test env — confirmed against main)
• Live cluster: obol sell http demo-hello --health-path /api/tags ... then buy.py buy demo-hello --endpoint http://traefik.../services/demo-hello/api/tags ... and confirm paid/demo returns 200

Coordination

Bundle PR #536 touches the same buy.py script area. This is a small, surgical change inside _normalize_endpoint; conflict resolution post-merge of #536 should be a quick rebase.