dom: move the native fetch() to RuntimeContext to decrease the async handles

[yorkie]

No description provided.

[Copilot]

Pull Request Overview

This PR refactors the resource fetching mechanism by moving fetch functionality from individual HTMLElements to a centralized RuntimeContext, eliminating redundant async handles and improving thread safety for resource loading operations.

Key changes include:

• Consolidation of thread-safe fetch operations into RuntimeContext
• Removal of individual async handles from HTMLElement instances
• Simplification of image loading methods

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/client/layout/layout_object.cpp	Updates method call to use new unified fetch API
src/client/layout/layout_image.cpp	Simplifies image loading by removing async variant
src/client/html/html_image_element.hpp	Removes async image loading method declaration
src/client/html/html_image_element.cpp	Consolidates image loading logic into single method
src/client/html/html_element.hpp	Removes thread-safe fetch methods and related infrastructure
src/client/html/html_element.cpp	Removes async handle management and processing logic
src/client/dom/runtime_context.hpp	Adds centralized fetch request management
src/client/dom/runtime_context.cpp	Implements unified fetch request handling with thread safety

[Copilot]

Same issue as above - using unique_lock with shared_mutex may be overly restrictive. Since this method modifies the deque, unique_lock is correct here, but consider if a regular mutex would be more appropriate for this use case.