fix(agent): enable MCP file preview via doc_id (#15399)

## Summary
MCP-wrapped agents could only force-download files looked up by
`doc_id`. This adds an explicit preview path and inline response headers
for previewable file types.

- **New** `GET /api/v1/agents/attachments/{attachment_id}/preview` —
inline preview for PDFs, images, and other safe types (pass `ext` and/or
`mime_type`)
- **Improved** `GET /api/v1/documents/{doc_id}/preview` — sets inline
disposition using the document filename
- **Improved** attachment download routing — resolves `mime_type` /
`ext` query params (no default `markdown`), supports
`disposition=inline`
- **DocGenerator output** — includes URL-encoded `preview_url` for MCP
clients
- **Legacy `/document/download/...` aliases** — still use download
semantics; MCP clients should call `/preview` explicitly

Fixes #15398

## Test plan
- [x] `pytest test/unit_test/api/utils/test_file_response_headers.py`
(6/6)

---------

Co-authored-by: MkDev11 <mkdev11@users.noreply.github.com>
Co-authored-by: Cursor <cursoragent@cursor.com>
Co-authored-by: Ling Qin <qinling0210@163.com>

test/unit_test/api/apps/restful_apis/test_attachment_download_missing_blob.py

@@ -47,6 +47,11 @@ class _LenientModule(ModuleType):
         return lambda *_a, **_k: None
 
 
+class _Headers(dict):
+    def set(self, key, value):
+        self[key] = value
+
+
 def _stub(monkeypatch, name, **attrs):
     mod = _LenientModule(name)
     for key, value in attrs.items():
@@ -61,7 +66,7 @@ def _load_agent_api(monkeypatch, *, storage_get):
     async def _make_response(payload):
         if payload is None:
             raise TypeError("response value cannot be None")
-        return SimpleNamespace(payload=payload, headers={})
+        return SimpleNamespace(payload=payload, headers=_Headers())
 
     _stub(
         monkeypatch,