feat(agent): add PLATFORM_HINTS for matrix, mattermost, and feishu (#14428)

* feat(agent): add PLATFORM_HINTS for matrix, mattermost, and feishu

These platform adapters fully support media delivery (send_image,
send_document, send_voice, send_video) but were missing from
PLATFORM_HINTS, leaving agents unaware of their platform context,
markdown rendering, and MEDIA: tag support.

Salvaged from PR #7370 by Rutimka — wecom excluded since main already
has a more detailed version.

Co-Authored-By: Marco Rutsch <marco@rutimka.de>

* test: add missing Markdown assertion for feishu platform hint

---------

Co-authored-by: Marco Rutsch <marco@rutimka.de>

agent/prompt_builder.py

@@ -370,6 +370,32 @@ PLATFORM_HINTS = {
         "MEDIA:/absolute/path/to/file in your response. Images (.jpg, .png, "
         ".heic) appear as photos and other files arrive as attachments."
     ),
+    "mattermost": (
+        "You are in a Mattermost workspace communicating with your user. "
+        "Mattermost renders standard Markdown — headings, bold, italic, code "
+        "blocks, and tables all work. "
+        "You can send media files natively: include MEDIA:/absolute/path/to/file "
+        "in your response. Images (.jpg, .png, .webp) are uploaded as photo "
+        "attachments, audio and video as file attachments. "
+        "Image URLs in markdown format ![alt](url) are rendered as inline previews automatically."
+    ),
+    "matrix": (
+        "You are in a Matrix room communicating with your user. "
+        "Matrix renders Markdown — bold, italic, code blocks, and links work; "
+        "the adapter converts your Markdown to HTML for rich display. "
+        "You can send media files natively: include MEDIA:/absolute/path/to/file "
+        "in your response. Images (.jpg, .png, .webp) are sent as inline photos, "
+        "audio (.ogg, .mp3) as voice/audio messages, video (.mp4) inline, "
+        "and other files as downloadable attachments."
+    ),
+    "feishu": (
+        "You are in a Feishu (Lark) workspace communicating with your user. "
+        "Feishu renders Markdown in messages — bold, italic, code blocks, and "
+        "links are supported. "
+        "You can send media files natively: include MEDIA:/absolute/path/to/file "
+        "in your response. Images (.jpg, .png, .webp) are uploaded and displayed "
+        "inline, audio files as voice messages, and other files as attachments."
+    ),
     "weixin": (
         "You are on Weixin/WeChat. Markdown formatting is supported, so you may use it when "
         "it improves readability, but keep the message compact and chat-friendly. You can send media files natively: "

tests/agent/test_prompt_builder.py

@@ -807,6 +807,24 @@ class TestPromptBuilderConstants:
         # check that this test is calibrated correctly).
         assert "include MEDIA:" in PLATFORM_HINTS["telegram"]
 
+    def test_platform_hints_mattermost(self):
+        hint = PLATFORM_HINTS["mattermost"]
+        assert "Mattermost" in hint
+        assert "MEDIA:" in hint
+        assert "Markdown" in hint
+
+    def test_platform_hints_matrix(self):
+        hint = PLATFORM_HINTS["matrix"]
+        assert "Matrix" in hint
+        assert "MEDIA:" in hint
+        assert "Markdown" in hint
+
+    def test_platform_hints_feishu(self):
+        hint = PLATFORM_HINTS["feishu"]
+        assert "Feishu" in hint
+        assert "MEDIA:" in hint
+        assert "Markdown" in hint
+
 
 # =========================================================================
 # Environment hints