Added offset and count parameters to pathsend extension

This extends the http.response.pathsend extension to support partial
file sends by adding optional offset and count parameters, enabling
HTTP Range request support (RFC 7233) and efficient seeking in large
media files.

The parameters mirror the existing http.response.zerocopysend API:
- offset: byte position to start reading from (default: beginning)
- count: number of bytes to send (default: to end of file)

This allows ASGI frameworks to implement range requests without
handling file descriptors directly, offloading the operation to
the server while maintaining pathsend's simpler API.

Like zerocopysend, these parameters are optional for applications
(with sensible defaults), but servers that advertise pathsend support
are expected to handle all documented parameters including offset
and count.

The application remains responsible for setting appropriate headers
(Content-Range, Content-Length, Accept-Ranges) in the response.

Fixes #469
Related: emmett-framework/granian#157

Author: matthewgrossman

asgiref/typing.py

@@ -140,6 +140,8 @@ class HTTPResponseTrailersEvent(TypedDict):
 class HTTPResponsePathsendEvent(TypedDict):
     type: Literal["http.response.pathsend"]
     path: str
+    offset: NotRequired[int]
+    count: NotRequired[int]
 
 
 class HTTPServerPushEvent(TypedDict):

docs/extensions.rst

@@ -160,6 +160,14 @@ Keys:
 * ``path`` (*Unicode string*): The string representation of the absolute
   file path to be sent by the server, platform specific.
 
+* ``offset`` (*int*): Optional. If this value exists, it will specify
+  the offset at which the server starts to read data from ``path``.
+  Otherwise, it will be read from the beginning of the file.
+
+* ``count`` (*int*): Optional. ``count`` is the number of bytes to
+  copy from the file. If omitted, the file will be read from the
+  offset (or beginning) until its end.
+
 The ASGI application itself is responsible to send the relevant headers
 in the *Response Start* message, like the ``Content-Type`` and
 ``Content-Length`` headers for the file to be sent.