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>

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