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.

Capability Signaling:
Servers that support offset and count parameters MUST advertise this
by setting {"ranges": True} in the pathsend extension scope dict.
Applications should check for this flag before using these parameters.
This provides a clear upgrade path from basic pathsend to range-capable
pathsend without ambiguity.

The application remains responsible for:
- Checking the "ranges" capability flag
- Parsing Range request headers
- Setting appropriate response headers (Content-Range, Content-Length,
  Accept-Ranges, status 206)

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

@@ -146,6 +146,22 @@ ASGI servers that implement this extension will provide
         },
     }
 
+Servers that support the ``offset`` and ``count`` parameters for range
+requests must advertise this capability via::
+
+    "scope": {
+        ...
+        "extensions": {
+            "http.response.pathsend": {
+                "ranges": True,
+            },
+        },
+    }
+
+Applications should check for the presence of ``ranges`` before using
+``offset`` and ``count`` parameters. Servers that advertise
+``"ranges": True`` must handle these parameters correctly.
+
 The ASGI framework can initiate a path-send by sending a message with
 the following keys. This message can be sent at any time after the
 *Response Start* message, and cannot be mixed with ``http.response.body``.
@@ -160,6 +176,14 @@ 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.