screenshot webhooks: Add support for multipart/form-data fixtures.

Add a common function for webhooks to convert multipart strings to dict.
This facilitates loading a multipart/form-data fixture as a file string,
and converting it.

This will allow testing integrations that use multipart/form-data,
and generating their example screenshots using a script.

Note that this only supports text fields, accommodation for binary files
is not included at the moment.

zerver/lib/webhooks/common.py

@@ -249,3 +249,27 @@ def unix_milliseconds_to_timestamp(milliseconds: Any, webhook: str) -> datetime:
         raise JsonableError(
             _("The {webhook} webhook expects time in milliseconds.").format(webhook=webhook)
         )
+
+
+def parse_multipart_string(body: str) -> dict[str, str]:
+    """
+    Converts multipart/form-data string (fixture) to dict
+    """
+    boundary = body.split("\n")[0][2:]
+    parts = body.split(f"--{boundary}")
+
+    data = {}
+    for part in parts:
+        if part.strip() in ["", "--"]:
+            continue
+
+        headers, body = part.split("\n\n", 1)
+        body = body.removesuffix("\n--")
+
+        content_disposition = next(
+            (line for line in headers.splitlines() if "Content-Disposition" in line), ""
+        )
+        field_name = content_disposition.split('name="')[1].split('"')[0]
+        data[field_name] = body
+
+    return data