merge_pr_58752

Prompt API: Reject system messages appended after others

Reject appending system messages when sessions have [pending] context.
Run ValidateInput on prompt and append; track imminent pending usage.
Update exception message. Add WPT coverage.

Fixed: 495843316
Change-Id: I77dfb450719e86041c3e1ebea83f0511c3db4e43
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7698599
Auto-Submit: Mike Wasserman msw@chromium.org
Commit-Queue: Mike Wasserman msw@chromium.org
Reviewed-by: Jingyun Liu jingyun@google.com
Cr-Commit-Position: refs/heads/main@{#1605041}