mcp: add Extensions field to capabilities per SEP-2133 (#794)

This implements
[SEP-2133](https://modelcontextprotocol.io/community/seps/2133-extensions)
and resolves #777.

## Summary of changes

- Add `Extensions` field to `ServerCapabilities` and
`ClientCapabilities` structs per
[SEP-2133](https://modelcontextprotocol.io/community/seps/2133-extensions)
- Add `AddExtension` helper methods on both types that normalize nil
settings to empty maps.
- [The spec requires an object, not
null](https://modelcontextprotocol.io/community/seps/2133-extensions#negotiation:~:text=an%20empty%20object%20indicates%20no%20settings.)
- Include shallow-copy support via `maps.Clone` in the existing
`clone()` methods
  - also include shallow-copy support for `Experimental` field.
- Add comprehensive tests: unit tests for nil-safety, capability
preservation, and over-the-wire round-trip

## What's tested

- [x] `go test ./mcp/...` passes
- [x] Server conformance tests pass (30 scenarios, 40 assertions, 0
failures)
- [x] Client conformance tests match existing baseline (auth scenarios
are known failures)

mcp/client_test.go

@@ -402,6 +402,26 @@ func TestClientCapabilities(t *testing.T) {
 				RootsV2:      &RootCapabilities{ListChanged: true},
 			},
 		},
+		{
+			name:            "extensions preserved",
+			configureClient: func(s *Client) {},
+			clientOpts: func() ClientOptions {
+				caps := &ClientCapabilities{
+					RootsV2: &RootCapabilities{ListChanged: true},
+				}
+				caps.AddExtension("io.example/ext1", map[string]any{"key": "value"})
+				caps.AddExtension("io.example/ext2", nil)
+				return ClientOptions{Capabilities: caps}
+			}(),
+			wantCapabilities: &ClientCapabilities{
+				Extensions: map[string]any{
+					"io.example/ext1": map[string]any{"key": "value"},
+					"io.example/ext2": map[string]any{},
+				},
+				Roots:   RootCapabilities{ListChanged: true},
+				RootsV2: &RootCapabilities{ListChanged: true},
+			},
+		},
 	}
 
 	for _, tc := range testCases {
@@ -469,6 +489,23 @@ func TestClientCapabilitiesOverWire(t *testing.T) {
 			},
 			wantCapabilities: &ClientCapabilities{},
 		},
+		{
+			name: "Extensions over wire",
+			clientOpts: func() *ClientOptions {
+				caps := &ClientCapabilities{
+					RootsV2: &RootCapabilities{ListChanged: true},
+				}
+				caps.AddExtension("io.example/ext", map[string]any{"key": "value"})
+				return &ClientOptions{Capabilities: caps}
+			}(),
+			wantCapabilities: &ClientCapabilities{
+				Extensions: map[string]any{
+					"io.example/ext": map[string]any{"key": "value"},
+				},
+				Roots:   RootCapabilities{ListChanged: true},
+				RootsV2: &RootCapabilities{ListChanged: true},
+			},
+		},
 	}
 
 	for _, tc := range testCases {