AI: Migrate executeJavaScript tool to registry for AIv2

This change migrates the executeJavaScript tool from an inline
declaration in StylingAgent to the centralized ToolRegistry. This is a
prerequisite for achieving full parity in the new skill-based V2 agent
architecture (AiAgent2) without relying on deprecated prototypes.

To support side-effect-heavy tools in V2, we:
1. Update the Tool and AiAgent2 contracts to forward approval options
   (FunctionHandlerOptions) to tool handlers.
2. Inject conversation-scoped state (ChangeManager and ExtensionScope)
   into the ToolContext so stateless registry tools can safely sandbox
   and track page modifications at runtime.

Both StylingAgent and AiAgent2 now dynamically load the tool from the
registry, ensuring seamless backward compatibility for V1 while making
the capability available to the V2 styling skill.

Note: The Accessibility (a11y) Agent has not been migrated to the
registry tool yet, creating temporary duplication of the
executeJavaScript function declaration in agents/ExecuteJavascript.ts.
This will be de-duplicated in a subsequent CL as part of the Phase 3
migration.

Bug: 510206549
Change-Id: I84a2e003edc1f489b7b9f51a759b9b847221aa6e
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/7902306
Reviewed-by: Kim-Anh Tran <kimanh@chromium.org>
Commit-Queue: Jack Franklin <jacktfranklin@chromium.org>