fix: 4 bugs found in full E2E usage testing

Bug 1: Reddit 403 due to short User-Agent in proxy test
  Root cause: configure proxy test used 'Mozilla/5.0' (too short, Reddit rejects)
  Fix: use full UA string matching the channel implementation

Bug 2: Reddit URL parsing broke on trailing slashes
  Root cause: url.rstrip('/') + '.json' mangled URLs with query params
  Fix: properly parse URL, clean path, reconstruct json URL

Bug 3: Reddit 403 without proxy showed raw HTTP error
  Root cause: no error handling for 403/429 responses
  Fix: friendly message suggesting proxy setup + Exa search alternative

Bug 4: XiaoHongShu without cookie showed Jina Reader 451 error
  Root cause: fallback to Jina Reader which can't access XHS (legal block)
  Fix: show clear message about needing cookies with setup instructions

Bug 5: Empty search query caused raw 422 API error
  Root cause: no input validation before API call
  Fix: check for empty query, show friendly message

All 36 unit tests passing.

agent_eyes/channels/xiaohongshu.py

@@ -27,9 +27,14 @@ class XiaoHongShuChannel(Channel):
         cookie = config.get("xhs_cookie") if config else None
 
         if not cookie:
-            # Fallback to Jina Reader (works for some public notes)
-            from agent_eyes.channels.web import WebChannel
-            return await WebChannel().read(url, config)
+            return ReadResult(
+                title="XiaoHongShu",
+                content="⚠️ XiaoHongShu requires cookies to access.\n"
+                        "Set up: agent-eyes configure xhs-cookie \"YOUR_COOKIE_STRING\"\n"
+                        "How to get it: install Cookie-Editor extension → go to xiaohongshu.com → Export → Header String",
+                url=url,
+                platform="xiaohongshu",
+            )
 
         # Extract note ID from URL
         note_id = self._extract_note_id(url)