agent: Remove duplicated description from tool schema

[bennetbo]

Turns out we were including the description of a tool inside the schema again, which I don't think is needed.
Before:

LanguageModelRequestTool {
    name: "web_search",
    description: "Search the web for information using your query.\nUse this when you need real-time information, facts, or data that might not be in your training.\nResults will include snippets and links from relevant web pages.",
    input_schema: Object {
        "required": Array [
            String("query"),
        ],
        "description": String("Search the web for information using your query.\nUse this when you need real-time information, facts, or data that might not be in your training.\nResults will include snippets and links from relevant web pages."),
        "type": String("object"),
        "properties": Object {
            "query": Object {
                "description": String("The search term or question to query on the web."),
                "type": String("string"),
            },
        },
        "additionalProperties": Bool(false),
    },
    use_input_streaming: false,
},

After:

LanguageModelRequestTool {
    name: "web_search",
    description: "Search the web for information using your query.\nUse this when you need real-time information, facts, or data that might not be in your training.\nResults will include snippets and links from relevant web pages.",
    input_schema: Object {
        "required": Array [
            String("query"),
        ],
        "type": String("object"),
        "properties": Object {
            "query": Object {
                "description": String("The search term or question to query on the web."),
                "type": String("string"),
            },
        },
        "additionalProperties": Bool(false),
    },
    use_input_streaming: false,
},

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content is consistent with the UI/UX checklist
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Closes #45315

Release Notes:

• agent: Reduced amount of tokens consumed by tool descriptions