Connect declarative form tools to ActorTask::Pause [10/N]

This CL implements the mechanism to pause actor execution when a
declarative WebMCP form tool requires user interaction. This happens
when the `toolautosubmit` attribute is absent, and the renderer is
waiting for the user to click the submit button.

Key changes:
- Added the `ScriptToolHost` Mojo interface to allow the renderer to
  request execution pauses from the browser.
- Implemented `ModelContext::PauseExecution` in Blink to trigger this
  IPC.
- Updated `HTMLFormElement` to call `PauseExecution` when a declarative
  tool is invoked without auto-submission.
- Added `ActorScriptToolReceiver` in the browser process to receive the
  IPC and call `ActorTask::Pause(true)` for the corresponding task.

Bug: 475992364
Fixed: 477323944
Change-Id: I577c8af866bfe822f16b1fc35dbedf1cbebc7009
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7543077
Reviewed-by: Ben Greenstein <bengr@chromium.org>
Reviewed-by: Elly FJ <ellyjones@chromium.org>
Commit-Queue: Mason Freed <masonf@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1580338}