Add market_product_url helper for correct product URLs

market_url('/product/...') was missing the /<page_slug>/<market_slug>/
prefix required by the market app's route structure. New helper
market_product_url() resolves the prefix from request context
(g.post_slug/g.market_slug in market app, g.page_slug + market_place
in cart app).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

infrastructure/urls.py

@@ -43,6 +43,24 @@ def page_cart_url(page_slug: str, path: str = "/") -> str:
     return cart_url(f"/{page_slug}{path}")
 
 
+def market_product_url(product_slug: str, suffix: str = "", market_place=None) -> str:
+    """Build a market product URL with the correct page/market prefix.
+
+    Resolves the prefix from:
+      - market app context: g.post_slug + g.market_slug
+      - cart app context:   g.page_slug + market_place.slug
+    """
+    from quart import g
+
+    page_slug = getattr(g, "post_slug", None) or getattr(g, "page_slug", None)
+    ms = getattr(g, "market_slug", None) or (
+        getattr(market_place, "slug", None) if market_place else None
+    )
+    prefix = f"/{page_slug}/{ms}" if page_slug and ms else ""
+    tail = f"/{suffix}" if suffix else "/"
+    return market_url(f"{prefix}/product/{product_slug}{tail}")
+
+
 def login_url(next_url: str = "") -> str:
     if next_url:
         return coop_url(f"/auth/login/?next={quote(next_url, safe='')}")