fix: handle missing content-type header in Request.post_data_json

[Skn0tt]

Problem

Request.post_data_json indexed self.headers["content-type"], which raises KeyError when a POST request has a body but no content-type header:

KeyError: 'content-type'

The documented behavior is to parse form-urlencoded bodies when that content-type is set, and otherwise fall back to JSON parsing. A missing header should take the JSON fallback path. This matches the upstream JS implementation, where this.headers()['content-type'] is undefined-safe before the form-urlencoded branch.

Closes microsoft#3093

Fix

In playwright/_impl/_network.py, read the header defensively and guard the branch:

content_type = self.headers.get("content-type")
if content_type and "application/x-www-form-urlencoded" in content_type:
    return dict(parse.parse_qsl(post_data))

Tests

Added to tests/async/test_network.py:

• test_should_parse_the_json_post_data_when_content_type_header_is_missing — the regression guard. Uses request.headers.delete('content-type') to produce a request whose content-type key is genuinely absent (unlike setting it to '', which keeps an empty-valued key), then asserts post_data_json returns the parsed JSON. Verified to fail with KeyError on the unpatched code.
• test_should_return_post_data_for_put_requests — mirrors the upstream network-post-data suite for parity.

All existing network tests continue to pass.

Notes

• No public API surface change, so no codegen / api.json updates are needed.

[Skn0tt]

Wrong base repo — reopening against microsoft/playwright-python.