fix(telegram): route image documents (.png/.jpg/.webp/.gif) through vision pipeline

When users send images as documents (Telegram file picker), they were
rejected with "Unsupported document type" because SUPPORTED_DOCUMENT_TYPES
only includes text/office formats. Add SUPPORTED_IMAGE_DOCUMENT_TYPES
to base.py and handle them in telegram.py before the document check.

- Add SUPPORTED_IMAGE_DOCUMENT_TYPES constant to base.py
- Add MIME reverse-lookup for image types in telegram.py
- Route image documents through cache_image_from_bytes + vision pipeline
- Handle media groups for image documents

Closes: #20128, #18620

gateway/platforms/base.py

@@ -837,6 +837,26 @@ SUPPORTED_DOCUMENT_TYPES = {
 }
 
 
+# ---------------------------------------------------------------------------
+# Image document types
+#
+# Image extensions that platforms may deliver as "documents" rather than
+# native photo attachments (Telegram users uploading via the file picker,
+# clients that wrap stickers/screenshots as files, etc.). When we see one
+# of these, we route the bytes through the image cache and the normal
+# vision/photo handling path instead of rejecting them as unsupported
+# documents.
+# ---------------------------------------------------------------------------
+
+SUPPORTED_IMAGE_DOCUMENT_TYPES = {
+    ".jpg": "image/jpeg",
+    ".jpeg": "image/jpeg",
+    ".png": "image/png",
+    ".webp": "image/webp",
+    ".gif": "image/gif",
+}
+
+
 def get_document_cache_dir() -> Path:
     """Return the document cache directory, creating it if it doesn't exist."""
     DOCUMENT_CACHE_DIR.mkdir(parents=True, exist_ok=True)