Issue description

The WPT WebDriver BiDi client supports session.new through BidiSession.bidi_only(), but webdriver/tests/bidi/session/ currently has no conformance coverage for the command.

Tests should cover:

• the returned session ID and required capabilities;
• association of the requesting WebSocket with the new session before the success response is observable;
• reuse of the same unassociated connection after a request with invalid capabilities fails;
• a session not created response when session.new is sent on an already associated connection, without affecting the existing session;
• isolation of a BiDi-only session from classic HTTP commands; and
• creation of a usable replacement session after the previous session ends.

This is separate from #63211 and #63212, which deliberately do not cover session.new.

Specification:
https://w3c.github.io/webdriver-bidi/#command-session-new