dsh-web-search-toggle: Add a "Search-then-Answer" Web Search toggle for DSH sessions

**Introduction:** When developing agents using DeepSeek Harness (DSH), a common issue arises: models typically do not have internet access by default, or they only invoke `web_search` when they deem it necessary. If you require the responses in a specific session to include the latest web information, you typically have to repeatedly emphasize "search before answering" in every round of prompts. DSH's philosophy is "everything is a plugin," and such behaviors can be handled by plugins. The dsh-web-search-toggle introduced below is exactly such a plugin: it adds a "Web Search" toggle switch to the chat input toolbar. Once enabled, it forces the agent to call `web_search` before answering in every round of that session, following the official DeepSeek approach.