Should the "New Session" command return default values for capabilities that have not specified?

Follow-up from https://github.com/w3c/webdriver/pull/1812.

While implementing the user prompt handler changes for WebDriver BiDi for the beforeunload user prompt in Firefox I noticed that Firefox and Chrome return both the default value for unhandledPromptBehavior when the capability has not been specified for the New Session command. Safari has a different handling here and doesn't return it.

This question most likely also applies to other capabilities like setWindowRect and others. Some capabilities we use for matching and others not. So maybe we should only do that for those that are utilized for capability matching?

If we agree that we should return the default value, the proposal for a fix could look like https://github.com/w3c/webdriver/pull/1812/commits/6294a9544899330017951cbe392f42798a4f0fe1.

CC @jgraham, @OrKoN, @sadym-chromium, @gsnedders, @AutomatedTester, @shs96c, @jimevans.

Start with the WebDriver BiDi beforeunload prompt-handling context and the follow-up PR 1812, including commit 6294a9544899330017951cbe392f42798a4f0fe1. Determine whether New Session should return defaults such as unhandledPromptBehavior and setWindowRect when capabilities are omitted, and whether this applies only to capabilities used for matching. Done means the specification has a decided, consistent rule.