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>

agent/component/docs_generator.py

@@ -13,6 +13,7 @@ from xml.sax.saxutils import escape
 
 from agent.component.base import ComponentParamBase
 from api.utils.api_utils import timeout
+from api.utils.file_response import agent_attachment_preview_path
 from common import settings
 from common.misc_utils import get_uuid
 from .message import Message
@@ -136,6 +137,7 @@ class DocGenerator(Message, ABC):
                     "mime_type": mime_type,
                     "size": file_size,
                     "base64": file_base64,
+                    "preview_url": agent_attachment_preview_path(doc_id, ext=output_format, mime_type=mime_type),
                     "include_download_info_in_content": self._param.include_download_info_in_content,
                 }
                 self.set_output("doc_id", doc_id)