Prompt API: create AIAssistantFactory

Following the explainer, the prompt API session is now created from
ai.assistant.

This CL moves the session creation logic to AIAssistantFactory, and
creates the capabilities API to return the model availability and the
text session information.

The WPT is updated correspondingly.

Bug: 355967885
Change-Id: I8becdcefcc108cb37eb32604b1c5a7bbc88a5b4f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5782731
Reviewed-by: Jiacheng Guo <gjc@google.com>
Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Commit-Queue: Mingyu Lei <leimy@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1343391}