fix(gateway): add media download retry to Mattermost, Slack, and base cache (#3323)

* fix(gateway): add media download retry to Mattermost, Slack, and base cache

Media downloads on Mattermost and Slack fail permanently on transient
errors (timeouts, 429 rate limits, 5xx server errors). Telegram and
WhatsApp already have retry logic, but these platforms had single-attempt
downloads with hardcoded 30s timeouts.

Changes:
- base.py cache_image_from_url: add retry with exponential backoff
  (covers Signal and any platform using the shared cache helper)
- mattermost.py _send_media_url: retry on 429/5xx/timeout (3 attempts)
- slack.py _download_slack_file: retry on timeout/5xx (3 attempts)
- slack.py _download_slack_file_bytes: same retry pattern

* test: add tests for media download retry

---------

Co-authored-by: dieutx <dangtc94@gmail.com>

gateway/platforms/base.py

@@ -72,31 +72,51 @@ def cache_image_from_bytes(data: bytes, ext: str = ".jpg") -> str:
     return str(filepath)
 
 
-async def cache_image_from_url(url: str, ext: str = ".jpg") -> str:
+async def cache_image_from_url(url: str, ext: str = ".jpg", retries: int = 2) -> str:
     """
     Download an image from a URL and save it to the local cache.
 
-    Uses httpx for async download with a reasonable timeout.
+    Retries on transient failures (timeouts, 429, 5xx) with exponential
+    backoff so a single slow CDN response doesn't lose the media.
 
     Args:
         url: The HTTP/HTTPS URL to download from.
         ext: File extension including the dot (e.g. ".jpg", ".png").
+        retries: Number of retry attempts on transient failures.
 
     Returns:
         Absolute path to the cached image file as a string.
     """
+    import asyncio
     import httpx
+    import logging as _logging
+    _log = _logging.getLogger(__name__)
 
+    last_exc = None
     async with httpx.AsyncClient(timeout=30.0, follow_redirects=True) as client:
-        response = await client.get(
-            url,
-            headers={
-                "User-Agent": "Mozilla/5.0 (compatible; HermesAgent/1.0)",
-                "Accept": "image/*,*/*;q=0.8",
-            },
-        )
-        response.raise_for_status()
-        return cache_image_from_bytes(response.content, ext)
+        for attempt in range(retries + 1):
+            try:
+                response = await client.get(
+                    url,
+                    headers={
+                        "User-Agent": "Mozilla/5.0 (compatible; HermesAgent/1.0)",
+                        "Accept": "image/*,*/*;q=0.8",
+                    },
+                )
+                response.raise_for_status()
+                return cache_image_from_bytes(response.content, ext)
+            except (httpx.TimeoutException, httpx.HTTPStatusError) as exc:
+                last_exc = exc
+                if isinstance(exc, httpx.HTTPStatusError) and exc.response.status_code < 429:
+                    raise
+                if attempt < retries:
+                    wait = 1.5 * (attempt + 1)
+                    _log.debug("Media cache retry %d/%d for %s (%.1fs): %s",
+                               attempt + 1, retries, url[:80], wait, exc)
+                    await asyncio.sleep(wait)
+                    continue
+                raise
+    raise last_exc
 
 
 def cleanup_image_cache(max_age_hours: int = 24) -> int: