fix(prompt): scope api_server MEDIA: hint to actual interception behavior

Correction to the previous commit (PR #68402): the claim that api_server
never intercepts MEDIA: tags is inaccurate on current main.
_resolve_media_to_data_urls() (gateway/platforms/api_server.py) DOES
inline image MEDIA: tags (<=5MB, image extensions only) as base64 data
URLs on the four main endpoints (_handle_session_chat,
_handle_session_chat_stream, _handle_chat_completions, _handle_responses).

The real gaps elphamale's PR points at are narrower:
- the /v1/runs output path (_handle_runs) never calls the resolver;
- non-image filetypes are never resolved anywhere (_MEDIA_IMG_EXT is
  image-only).

Reword the hint to teach both halves: images via MEDIA: work on the
chat/completions/responses endpoints; non-image files and anything on
the runs endpoint must fall back to plain file paths in the response
text. Update the test to pin the scoped guidance instead of a blanket
prohibition.

agent/prompt_builder.py

@@ -885,12 +885,13 @@ PLATFORM_HINTS = {
         "assume plain text. No markdown formatting (no asterisks, bullets, headers, "
         "code fences). Treat this like a conversation, not a document. Keep responses "
         "brief and natural. "
-        "File/media delivery: do NOT emit MEDIA:/path tags in your response — "
-        "that convention is only intercepted on messaging platforms (Telegram, "
-        "WebUI, etc.); here it renders as literal, unusable text exposing a raw "
-        "host filesystem path to the caller. If a registered file-delivery tool "
-        "is available in your toolset, use it; otherwise there is no channel to "
-        "deliver a file on this platform."
+        "File/media delivery: images referenced as MEDIA:/absolute/path tags "
+        "(.png/.jpg/.jpeg/.gif/.webp/.bmp, up to 5MB) are inlined as base64 data "
+        "URLs in responses on the chat, completions, and responses endpoints. "
+        "Non-image files are NOT intercepted anywhere, and the runs endpoint "
+        "intercepts nothing — a MEDIA: tag there renders as literal text exposing "
+        "a raw host filesystem path. For those cases, state the plain file path "
+        "in your response text instead of a MEDIA: tag."
     ),
     "webui": (
         "You are in the Hermes WebUI, a browser-based chat interface. "

tests/agent/test_prompt_builder.py

@@ -1103,18 +1103,24 @@ class TestPromptBuilderConstants:
         hint = PLATFORM_HINTS["whatsapp_cloud"]
         assert "MEDIA:" in hint
 
-    def test_api_server_hint_forbids_media_tag(self):
-        """api_server has no MEDIA: interception (unlike telegram/webui/etc.,
-        which resolve it to an attachment or a validated/inlined data URL) —
-        _handle_runs never calls the resolver other api_server endpoints use.
-        Without an explicit prohibition, the model's general cross-platform
-        habit of using MEDIA:/path for file delivery (correct and taught for
-        several other platforms in this same dict) can surface here too, with
-        no interception to catch it: the tag renders as literal text in the
-        response, exposing a raw host filesystem path to the API caller."""
+    def test_api_server_hint_scopes_media_tag_guidance(self):
+        """api_server MEDIA: interception is partial (#68402, corrected):
+        _resolve_media_to_data_urls (gateway/platforms/api_server.py) inlines
+        small image MEDIA: tags as base64 data URLs on the chat, completions,
+        and responses endpoints — but non-image files are never resolved
+        (_MEDIA_IMG_EXT is image-only) and the /v1/runs handler never calls
+        the resolver at all. The hint must teach BOTH halves: images work via
+        MEDIA:, everything else needs a plain path in the response text."""
         hint = PLATFORM_HINTS["api_server"]
+        # Images ARE intercepted: inlined as data URLs.
         assert "MEDIA:" in hint
-        assert "not" in hint.lower()
+        assert "inlined" in hint.lower()
+        assert "data" in hint.lower()  # data URLs
+        # The gaps: non-image files and the runs endpoint.
+        assert "non-image" in hint.lower()
+        assert "runs" in hint.lower()
+        # Fallback guidance: plain file path in the response text.
+        assert "plain" in hint.lower()
 
     def test_markdown_converting_platform_hints_do_not_forbid_markdown(self):
         """#12224 — WhatsApp (Baileys) and Signal adapters actively convert