test: add comprehensive URL construction edge case tests

- Trailing slash handling (baseURL with /)
- Multiple query parameters
- URL-encoded special characters
- Empty path/query handling
- Boundary conditions (nil/empty rpcReqs)
- Production-like Kubernetes service URLs

All tests pass ✅ - URL contract is solid.

Author: odysseus0

backend_url_edge_cases_test.go

@@ -0,0 +1,172 @@
+package proxyd
+
+import (
+	"context"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+// TestBackendURLEdgeCases covers all the tricky slash/query edge cases
+func TestBackendURLEdgeCases(t *testing.T) {
+	tests := []struct {
+		name     string
+		baseURL  string
+		method   string
+		path     string
+		query    string
+		expected string
+	}{
+		// === SLASH EDGE CASES ===
+		{
+			name:     "base URL with trailing slash + path",
+			baseURL:  "http://backend:8080/",
+			method:   "eth_sendRawTransaction",
+			path:     "/fast",
+			query:    "",
+			expected: "http://backend:8080/fast", // Should NOT be //fast
+		},
+		{
+			name:     "base URL with trailing slash + path + query",
+			baseURL:  "http://backend:8080/",
+			method:   "eth_sendRawTransaction",
+			path:     "/fast",
+			query:    "hint=hash",
+			expected: "http://backend:8080/fast?hint=hash",
+		},
+		{
+			name:     "path is just slash with query",
+			baseURL:  "http://backend:8080",
+			method:   "eth_sendRawTransaction",
+			path:     "/",
+			query:    "hint=hash",
+			expected: "http://backend:8080?hint=hash", // Should NOT add slash before ?
+		},
+
+		// === QUERY STRING EDGE CASES ===
+		{
+			name:     "multiple query parameters",
+			baseURL:  "http://backend:8080",
+			method:   "eth_sendRawTransaction",
+			path:     "/fast",
+			query:    "hint=hash&builder=builder1&origin=wallet",
+			expected: "http://backend:8080/fast?hint=hash&builder=builder1&origin=wallet",
+		},
+		{
+			name:     "query with URL-encoded special chars",
+			baseURL:  "http://backend:8080",
+			method:   "eth_sendRawTransaction",
+			path:     "/fast",
+			query:    "hint=0x1234%20test&url=https%3A%2F%2Fexample.com",
+			expected: "http://backend:8080/fast?hint=0x1234%20test&url=https%3A%2F%2Fexample.com",
+		},
+		{
+			name:     "query with equals in value",
+			baseURL:  "http://backend:8080",
+			method:   "eth_sendRawTransaction",
+			path:     "/fast",
+			query:    "signature=0xabc=def",
+			expected: "http://backend:8080/fast?signature=0xabc=def",
+		},
+
+		// === EMPTY/MISSING EDGE CASES ===
+		{
+			name:     "empty path string",
+			baseURL:  "http://backend:8080",
+			method:   "eth_sendRawTransaction",
+			path:     "",
+			query:    "hint=hash",
+			expected: "http://backend:8080?hint=hash", // Empty path should be treated like "/"
+		},
+		{
+			name:     "empty query string",
+			baseURL:  "http://backend:8080",
+			method:   "eth_sendRawTransaction",
+			path:     "/fast",
+			query:    "",
+			expected: "http://backend:8080/fast", // Should NOT add ? for empty query
+		},
+		{
+			name:     "both path and query empty",
+			baseURL:  "http://backend:8080",
+			method:   "eth_sendRawTransaction",
+			path:     "",
+			query:    "",
+			expected: "http://backend:8080",
+		},
+
+		// === METHOD FILTERING ===
+		{
+			name:     "non-eth_sendRawTransaction ignores everything",
+			baseURL:  "http://backend:8080",
+			method:   "eth_call",
+			path:     "/fast",
+			query:    "hint=hash&builder=builder1",
+			expected: "http://backend:8080", // Should completely ignore path and query
+		},
+
+		// === PRODUCTION-LIKE SCENARIOS ===
+		{
+			name:     "real production URL with everything",
+			baseURL:  "http://rpc-endpoint.flashbots.svc.cluster.local:8080",
+			method:   "eth_sendRawTransaction",
+			path:     "/fast",
+			query:    "hint=0xabcdef1234567890&builder=flashbots&origin=metamask",
+			expected: "http://rpc-endpoint.flashbots.svc.cluster.local:8080/fast?hint=0xabcdef1234567890&builder=flashbots&origin=metamask",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			ctx := context.Background()
+			ctx = context.WithValue(ctx, ContextKeyPath, tt.path)
+			ctx = context.WithValue(ctx, ContextKeyRawQuery, tt.query)
+
+			rpcReqs := []*RPCReq{{Method: tt.method}}
+			backendURL := buildBackendURL(tt.baseURL, rpcReqs, ctx)
+
+			assert.Equal(t, tt.expected, backendURL, "URL mismatch for case: %s", tt.name)
+		})
+	}
+}
+
+// TestBackendURLBoundaryConditions tests weird inputs that shouldn't happen but might
+func TestBackendURLBoundaryConditions(t *testing.T) {
+	tests := []struct {
+		name     string
+		baseURL  string
+		rpcReqs  []*RPCReq
+		path     string
+		query    string
+		expected string
+	}{
+		{
+			name:     "empty rpcReqs array",
+			baseURL:  "http://backend:8080",
+			rpcReqs:  []*RPCReq{},
+			path:     "/fast",
+			query:    "hint=hash",
+			expected: "http://backend:8080", // Should not panic, just return base
+		},
+		{
+			name:     "nil rpcReqs array",
+			baseURL:  "http://backend:8080",
+			rpcReqs:  nil,
+			path:     "/fast",
+			query:    "hint=hash",
+			expected: "http://backend:8080",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			ctx := context.Background()
+			ctx = context.WithValue(ctx, ContextKeyPath, tt.path)
+			ctx = context.WithValue(ctx, ContextKeyRawQuery, tt.query)
+
+			// Should not panic
+			backendURL := buildBackendURL(tt.baseURL, tt.rpcReqs, ctx)
+			assert.Equal(t, tt.expected, backendURL)
+		})
+	}
+}