mcp: fix race condition in ServerSession.startKeepalive

[begelundmuller]

This PR fixes a race condition where ServerSession.keepaliveCancel was accessed in Close concurrently with its initialization in ServerSession.startKeepalive. It also removes a now redundant test.

The fix moves keepalive initialization into ServerSession.Connect similar to the implementation in ClientSession.Connect. This is safe because the MCP spec allows keepalive pings before the initialization messages. From the MCP spec (emphasis added):

The client SHOULD NOT send requests other than pings before the server has responded to the initialize request.

The PR includes a reproducer test case, which fails with the following error without this fix:

> go test -count=10 -race -run ^TestStreamableStatelessKeepaliveRace$ github.com/modelcontextprotocol/go-sdk/mcp                                                                                                                          8s
==================
WARNING: DATA RACE
Write at 0x00c00007fbf0 by goroutine 1095:
  github.com/modelcontextprotocol/go-sdk/mcp.startKeepalive()
      /Users/benjamin/Documents/go-sdk/mcp/shared.go:590 +0x58
  github.com/modelcontextprotocol/go-sdk/mcp.(*ServerSession).startKeepalive()
      /Users/benjamin/Documents/go-sdk/mcp/server.go:1526 +0x130
  github.com/modelcontextprotocol/go-sdk/mcp.(*ServerSession).initialized()
      /Users/benjamin/Documents/go-sdk/mcp/server.go:1059 +0xf8
  github.com/modelcontextprotocol/go-sdk/mcp.init.serverSessionMethod[go.shape.*uint8,go.shape.interface { GetMeta() map[string]interface {}; SetMeta(map[string]interface {}); github.com/modelcontextprotocol/go-sdk/mcp.isResult() }].func27()
      /Users/benjamin/Documents/go-sdk/mcp/shared.go:332 +0x98
  github.com/modelcontextprotocol/go-sdk/mcp.newServerMethodInfo[go.shape.*github.com/modelcontextprotocol/go-sdk/mcp.InitializedParams,go.shape.interface { GetMeta() map[string]interface {}; SetMeta(map[string]interface {}); github.com/modelcontextprotocol/go-sdk/mcp.isResult() },go.shape.struct { github.com/modelcontextprotocol/go-sdk/mcp.Meta "json:\"_meta,omitempty\"" }].func2()
      /Users/benjamin/Documents/go-sdk/mcp/shared.go:272 +0x84
  github.com/modelcontextprotocol/go-sdk/mcp.defaultReceivingMethodHandler[go.shape.*uint8]()
      /Users/benjamin/Documents/go-sdk/mcp/shared.go:152 +0xe8
  github.com/modelcontextprotocol/go-sdk/mcp.defaultReceivingMethodHandler[*github.com/modelcontextprotocol/go-sdk/mcp.ServerSession]()
      /Users/benjamin/Documents/go-sdk/mcp/shared.go:146 +0x60
  github.com/modelcontextprotocol/go-sdk/mcp.handleReceive[go.shape.*uint8]()
      /Users/benjamin/Documents/go-sdk/mcp/shared.go:169 +0x228
  github.com/modelcontextprotocol/go-sdk/mcp.(*ServerSession).handle()
      /Users/benjamin/Documents/go-sdk/mcp/server.go:1445 +0x370
  github.com/modelcontextprotocol/go-sdk/mcp.connect[go.shape.*uint8,go.shape.*uint8].func1.1()
      /Users/benjamin/Documents/go-sdk/mcp/transport.go:170 +0x5c
  github.com/modelcontextprotocol/go-sdk/internal/jsonrpc2.HandlerFunc.Handle()
      /Users/benjamin/Documents/go-sdk/internal/jsonrpc2/jsonrpc2.go:84 +0x48
  github.com/modelcontextprotocol/go-sdk/internal/jsonrpc2.(*Connection).handleAsync.func3()
      /Users/benjamin/Documents/go-sdk/internal/jsonrpc2/conn.go:717 +0xd8

Previous read at 0x00c00007fbf0 by goroutine 1089:
  github.com/modelcontextprotocol/go-sdk/mcp.(*ServerSession).Close()
      /Users/benjamin/Documents/go-sdk/mcp/server.go:1502 +0x34
  github.com/modelcontextprotocol/go-sdk/mcp.(*StreamableHTTPHandler).ServeHTTP.deferwrap1()
      /Users/benjamin/Documents/go-sdk/mcp/streamable.go:519 +0x34
  runtime.deferreturn()
      /opt/homebrew/Cellar/go/1.25.4/libexec/src/runtime/panic.go:589 +0x5c
  github.com/modelcontextprotocol/go-sdk/mcp.TestStreamableStatelessKeepaliveRace.mustNotPanic.func2()
      /Users/benjamin/Documents/go-sdk/mcp/streamable_test.go:2156 +0x94
  net/http.HandlerFunc.ServeHTTP()
      /opt/homebrew/Cellar/go/1.25.4/libexec/src/net/http/server.go:2322 +0x48
  net/http.serverHandler.ServeHTTP()
      /opt/homebrew/Cellar/go/1.25.4/libexec/src/net/http/server.go:3340 +0x270
  net/http.(*conn).serve()
      /opt/homebrew/Cellar/go/1.25.4/libexec/src/net/http/server.go:2109 +0x9b0
  net/http.(*Server).Serve.gowrap3()
      /opt/homebrew/Cellar/go/1.25.4/libexec/src/net/http/server.go:3493 +0x48

Goroutine 1095 (running) created at:
  github.com/modelcontextprotocol/go-sdk/internal/jsonrpc2.(*Connection).handleAsync()
      /Users/benjamin/Documents/go-sdk/internal/jsonrpc2/conn.go:715 +0x334
  github.com/modelcontextprotocol/go-sdk/internal/jsonrpc2.(*Connection).acceptRequest.func2.gowrap1()
      /Users/benjamin/Documents/go-sdk/internal/jsonrpc2/conn.go:676 +0x34

Goroutine 1089 (running) created at:
  net/http.(*Server).Serve()
      /opt/homebrew/Cellar/go/1.25.4/libexec/src/net/http/server.go:3493 +0x5dc
  net/http/httptest.(*Server).goServe.func1()
      /opt/homebrew/Cellar/go/1.25.4/libexec/src/net/http/httptest/server.go:311 +0x98
==================
--- FAIL: TestStreamableStatelessKeepaliveRace (0.14s)
    testing.go:1617: race detected during execution of test
FAIL
FAIL	github.com/modelcontextprotocol/go-sdk/mcp	1.727s
FAIL