mcp: support stateless streamable sessions (#277)

Support stateless streamable sessions by adding a GetSessionID function
to StreamableHTTPOptions. If GetSessionID returns "", the session is
stateless, and no validation is performed. This is implemented by
providing the session a trivial initialization state.

To implement this, some parts of #232 (distributed sessions) are copied
over, since they add an API for creating an already-initialized session.

In total, the following new API is added:
- StreamableHTTPOptions.GetSessionID
- ServerSessionOptions (a new parameter to Server.Connect)
- ServerSessionState
- ClientSessionOptions (a new parameter to Client.Connect, for symmetry)

For #10

mcp/client_list_test.go

@@ -38,7 +38,7 @@ func TestList(t *testing.T) {
 			}
 		})
 		t.Run("iterator", func(t *testing.T) {
-			testIterator(ctx, t, clientSession.Tools(ctx, nil), wantTools)
+			testIterator(t, clientSession.Tools(ctx, nil), wantTools)
 		})
 	})
 
@@ -60,7 +60,7 @@ func TestList(t *testing.T) {
 			}
 		})
 		t.Run("iterator", func(t *testing.T) {
-			testIterator(ctx, t, clientSession.Resources(ctx, nil), wantResources)
+			testIterator(t, clientSession.Resources(ctx, nil), wantResources)
 		})
 	})
 
@@ -81,7 +81,7 @@ func TestList(t *testing.T) {
 			}
 		})
 		t.Run("ResourceTemplatesIterator", func(t *testing.T) {
-			testIterator(ctx, t, clientSession.ResourceTemplates(ctx, nil), wantResourceTemplates)
+			testIterator(t, clientSession.ResourceTemplates(ctx, nil), wantResourceTemplates)
 		})
 	})
 
@@ -102,12 +102,12 @@ func TestList(t *testing.T) {
 			}
 		})
 		t.Run("iterator", func(t *testing.T) {
-			testIterator(ctx, t, clientSession.Prompts(ctx, nil), wantPrompts)
+			testIterator(t, clientSession.Prompts(ctx, nil), wantPrompts)
 		})
 	})
 }
 
-func testIterator[T any](ctx context.Context, t *testing.T, seq iter.Seq2[*T, error], want []*T) {
+func testIterator[T any](t *testing.T, seq iter.Seq2[*T, error], want []*T) {
 	t.Helper()
 	var got []*T
 	for x, err := range seq {