[proxy] Keep the Vertex model id out of the count-tokens method segment

Vertex hangs token counting off the model as its own path segment, and the
parser split the tail on the final colon alone. A count-tokens request
therefore reported its model as "claude-sonnet-5/count-tokens", which no
route claims, so the request denied as not-routable and the access log
recorded a model that does not exist.

Stop at the first "/" after the model id so the method segment stays out of
it, leaving the client free to price its context against the dedicated
endpoint instead of the billable inference one.

proxy/internal/middleware/builtin/llm_request_parser/middleware.go

@@ -257,6 +257,12 @@ func parseVertexPath(reqPath string) (vertexRequest, bool) {
 	if c := strings.LastIndex(rest, ":"); c >= 0 {
 		model, action = rest[:c], rest[c+1:]
 	}
+	// Token counting hangs off the model as its own path segment
+	// (".../models/{model}/count-tokens:rawPredict"), so anything past the
+	// first "/" belongs to the method rather than the model id.
+	if slash := strings.Index(model, "/"); slash >= 0 {
+		model = model[:slash]
+	}
 	model = llm.NormalizeVertexModel(model)
 	if model == "" {
 		return vertexRequest{}, false