feat: integrate tool fetching and calling with SSE transport

[zhuozl]

支持在网关采用 MCP 协议、后端服务使用 SSE 协议的场景下，实现对后端工具的获取与调用功能。

Summary by Sourcery

Add support for fetching and invoking backend tools over SSE transport in the MCP server.

New Features:

• Enable tool discovery via SSE backend protocol using FetchTools.
• Enable tool invocation over SSE protocol through CallTool with error handling when transport is missing or execution fails.

[sourcery-ai]

Reviewer's Guide

This PR integrates support for fetching and invoking backend tools over SSE in the MCP gateway scenario by retrieving the SSE transport from server state and delegating tool operations to it with proper error handling.

Sequence Diagram for Fetching Tools via SSE Transport

sequenceDiagram
    actor User
    participant MCPGateway
    participant State
    participant SSETransport
    participant BackendServiceSSE

    User->>MCPGateway: MCP Request (method: tools/list, protocol: SSE)
    MCPGateway->>State: GetTransport(conn.Meta().Prefix)
    State-->>MCPGateway: transport (SSETransport instance)
    alt transport is available
        MCPGateway->>SSETransport: FetchTools(context)
        activate SSETransport
        SSETransport->>BackendServiceSSE: Fetch tools over SSE
        BackendServiceSSE-->>SSETransport: Tools data
        deactivate SSETransport
        SSETransport-->>MCPGateway: tools
        MCPGateway-->>User: MCP Response (tools)
    else transport is nil OR FetchTools fails
        MCPGateway-->>User: MCP Response (Error: Failed to fetch tools)
    end

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Sequence Diagram for Calling a Tool via SSE Transport

sequenceDiagram
    actor User
    participant MCPGateway
    participant State
    participant SSETransport
    participant BackendServiceSSE

    User->>MCPGateway: MCP Request (method: tools/call, params, protocol: SSE)
    MCPGateway->>State: GetTransport(conn.Meta().Prefix)
    State-->>MCPGateway: transport (SSETransport instance)
    alt transport is available
        MCPGateway->>SSETransport: CallTool(context, params, requestInfo)
        activate SSETransport
        SSETransport->>BackendServiceSSE: Execute tool over SSE with params
        BackendServiceSSE-->>SSETransport: Tool result
        deactivate SSETransport
        SSETransport-->>MCPGateway: result
        MCPGateway-->>User: MCP Response (result)
    else transport is nil OR CallTool fails
        MCPGateway-->>User: MCP Response (Error: Tool execution error or config not found)
    end

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Class Diagram: Server Interaction with SSE Transport

classDiagram
    class Server {
        +state: State
        +handleMCPRequest(c gin.Context, req mcp.JSONRPCRequest, conn session.Conn)
    }
    class State {
        +GetTransport(prefix string) Transport
    }
    class Transport {
        <<Interface>>
        +FetchTools(ctx context.Context) []Tool
        +CallTool(ctx context.Context, params any, reqInfo any) any
    }
    class SSETransport {
        +FetchTools(ctx context.Context) []Tool
        +CallTool(ctx context.Context, params any, reqInfo any) any
    }

    Server o-- State : uses
    State ..> Transport : provides
    Transport <|.. SSETransport : implements

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File-Level Changes

Change	Details	Files
Reorder imports for consistency	Moved github.com/google/uuid import below mcp pkg imports	internal/core/streamable.go
Implement tool fetching over SSE	Retrieve transport via state.GetTransport Handle nil transport with protocol error Call transport.FetchTools and handle errors	internal/core/streamable.go
Implement tool invocation over SSE	Retrieve transport via state.GetTransport Handle missing transport with 404 protocol error Invoke transport.CallTool with merged request info Handle execution errors via sendToolExecutionError	internal/core/streamable.go

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[sourcery-ai]

Hey @zhuozl - I've reviewed your changes and they look great!

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