fix: raise Error instead of assert for user input validation

[Pavel Feldman (pavelfeldman)]

Several checks on user input used assert, so python -O stripped them. For example, request.post(url, data="", form={...}) then sent the request and dropped form without telling the caller. Without -O, these checks raised AssertionError, which isn't a Playwright Error.

This turns every user-facing assert in _impl into raise Error(...) and keeps the messages. That covers:

• APIRequestContext.fetch: the first argument must be a URL or a Request; only one of data / form / multipart; non-negative max_redirects / max_retries; form file payloads must have a bytes buffer.
• Serializing an evaluate argument: only str regex patterns are supported.
• Regex flags: only MULTILINE / IGNORECASE / DOTALL are supported.
• to_have_text / to_contain_text with a list: the expected value must be a list or tuple.

The asserts that check internal invariants (channel state, transport, _soft_scope nesting) are unchanged.

Tests: the existing tests for these messages now expect Error. They fail on main because it raises AssertionError, and they pass with this change, both normally and under python -O. The neighbouring fetch, evaluate, assertions and interception suites pass, async and sync.

Fixes: microsoft/playwright#43281