Add fragment client for coop app composition

Lightweight httpx-based client to fetch HTML fragments from coop apps
(nav-tree, auth-menu, cart-mini) using internal Docker URLs.

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

artdag_common/fragments.py

@@ -0,0 +1,91 @@
+"""Fragment client for fetching HTML fragments from coop apps.
+
+Lightweight httpx-based client (no Quart dependency) for Art-DAG to consume
+coop app fragments like nav-tree, auth-menu, and cart-mini.
+"""
+
+from __future__ import annotations
+
+import asyncio
+import logging
+import os
+from typing import Sequence
+
+import httpx
+
+log = logging.getLogger(__name__)
+
+FRAGMENT_HEADER = "X-Fragment-Request"
+
+_client: httpx.AsyncClient | None = None
+_DEFAULT_TIMEOUT = 2.0
+
+
+def _get_client() -> httpx.AsyncClient:
+    global _client
+    if _client is None or _client.is_closed:
+        _client = httpx.AsyncClient(
+            timeout=httpx.Timeout(_DEFAULT_TIMEOUT),
+            follow_redirects=False,
+        )
+    return _client
+
+
+def _internal_url(app_name: str) -> str:
+    """Resolve internal base URL for a coop app.
+
+    Looks up ``INTERNAL_URL_{APP}`` first, falls back to ``http://{app}:8000``.
+    """
+    env_key = f"INTERNAL_URL_{app_name.upper()}"
+    return os.getenv(env_key, f"http://{app_name}:8000").rstrip("/")
+
+
+async def fetch_fragment(
+    app_name: str,
+    fragment_type: str,
+    *,
+    params: dict | None = None,
+    timeout: float = _DEFAULT_TIMEOUT,
+    required: bool = False,
+) -> str:
+    """Fetch an HTML fragment from a coop app.
+
+    Returns empty string on failure by default (required=False).
+    """
+    base = _internal_url(app_name)
+    url = f"{base}/internal/fragments/{fragment_type}"
+    try:
+        resp = await _get_client().get(
+            url,
+            params=params,
+            headers={FRAGMENT_HEADER: "1"},
+            timeout=timeout,
+        )
+        if resp.status_code == 200:
+            return resp.text
+        msg = f"Fragment {app_name}/{fragment_type} returned {resp.status_code}"
+        log.warning(msg)
+        if required:
+            raise RuntimeError(msg)
+        return ""
+    except RuntimeError:
+        raise
+    except Exception as exc:
+        msg = f"Fragment {app_name}/{fragment_type} failed: {exc}"
+        log.warning(msg)
+        if required:
+            raise RuntimeError(msg) from exc
+        return ""
+
+
+async def fetch_fragments(
+    requests: Sequence[tuple[str, str, dict | None]],
+    *,
+    timeout: float = _DEFAULT_TIMEOUT,
+    required: bool = False,
+) -> list[str]:
+    """Fetch multiple fragments concurrently."""
+    return list(await asyncio.gather(*(
+        fetch_fragment(app, ftype, params=params, timeout=timeout, required=required)
+        for app, ftype, params in requests
+    )))