Add close_sse_stream callback to tool context (SEP-1699 follow-up)

[felixweinberger]

Summary

This PR adds a close_sse_stream callback to the tool context, addressing feedback from findleyr on the TypeScript SDK PR. This is a follow-up to #1654 (SEP-1699 SSE polling).

The key improvements:

• Reduced coupling: Tools no longer need direct access to session_manager - the callback is injected into the context
• Event store awareness: Callback is None if no event store is configured (so tools don't need to check this themselves)
• Per-call retry interval: Each call to close_sse_stream() can specify its own retry interval in milliseconds

API Usage

FastMCP (high-level):

@mcp.tool()
async def long_running_task(ctx: Context) -> str:
    await ctx.info("Starting task...")
    
    # Close SSE stream to trigger polling behavior
    # Callback is None if not on streamable HTTP transport or no event store
    if ctx.close_sse_stream:
        await ctx.close_sse_stream(retry_interval=3000)  # Reconnect after 3s
    
    await asyncio.sleep(1)
    return "Task completed"

Lowlevel Server:

@app.call_tool()
async def call_tool(name: str, arguments: dict) -> list[types.ContentBlock]:
    ctx = app.request_context
    
    if ctx.close_sse_stream:
        await ctx.close_sse_stream(retry_interval=5000)
    
    return [types.TextContent(type="text", text="Done")]

Changes

• src/mcp/shared/context.py: Added CloseSSEStreamCallback type and close_sse_stream field to RequestContext
• src/mcp/shared/message.py: Added close_sse_stream field to ServerMessageMetadata
• src/mcp/server/fastmcp/server.py: Added close_sse_stream property to Context class
• src/mcp/server/streamable_http.py: Added callback factory and retry_interval parameter
• src/mcp/server/streamable_http_manager.py: Pass through retry_interval parameter
• src/mcp/server/lowlevel/server.py: Extract callback from metadata into RequestContext
• Updated examples to use the new callback API

Test plan

• All existing tests pass (772 tests)
• Manual testing with everything-server test_reconnection tool