fix(security): patch path traversal, size bypass, and prompt injection in document processing

- Sanitize filenames in cache_document_from_bytes to prevent path traversal (strip directory components, null bytes, resolve check)
- Reject documents with None file_size instead of silently allowing download
- Cap text file injection at 100 KB to prevent oversized prompt payloads
- Sanitize display_name in run.py context notes to block prompt injection via filenames
- Add 35 unit tests covering document cache utilities and Telegram document handling

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

gateway/run.py

@@ -757,6 +757,9 @@ class GatewayRunner:
                 # Format: doc_<12hex>_<original_filename>
                 parts = basename.split("_", 2)
                 display_name = parts[2] if len(parts) >= 3 else basename
+                # Sanitize to prevent prompt injection via filenames
+                import re as _re
+                display_name = _re.sub(r'[^\w.\- ]', '_', display_name)
 
                 if mtype.startswith("text/"):
                     context_note = (