internal/mcp: update protocol versions

Update protocol versions to reflect that 2025-03-26 is supported, and
add server logic to negotiate the supported version with the client.

Update the client to set the protocol version, fixing what was probably
a spec violation.

+tests

Change-Id: I564b591b3daf018dd54f5c7e5d8f2d2b1e4ff502
Reviewed-on: https://go-review.googlesource.com/c/tools/+/682516
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>

mcp/testdata/conformance/server/version-latest.txtar

@@ -0,0 +1,40 @@
+Check that when the client sends an arbitrary later version, the server
+response with its latest supported version.
+
+-- client --
+{
+  "jsonrpc": "2.0",
+  "id": 1,
+  "method": "initialize",
+  "params": {
+    "protocolVersion": "2029-01-01",
+    "capabilities": {},
+    "clientInfo": { "name": "ExampleClient", "version": "1.0.0" }
+  }
+}
+-- server --
+{
+	"jsonrpc": "2.0",
+	"id": 1,
+	"result": {
+		"_meta": {},
+		"capabilities": {
+			"completions": {},
+			"logging": {},
+			"prompts": {
+				"listChanged": true
+			},
+			"resources": {
+				"listChanged": true
+			},
+			"tools": {
+				"listChanged": true
+			}
+		},
+		"protocolVersion": "2025-03-26",
+		"serverInfo": {
+			"name": "testServer",
+			"version": "v1.0.0"
+		}
+	}
+}

mcp/testdata/conformance/server/version-older.txtar

@@ -0,0 +1,40 @@
+Check that when the client sends an older version, the server indicates
+support.
+
+-- client --
+{
+  "jsonrpc": "2.0",
+  "id": 1,
+  "method": "initialize",
+  "params": {
+    "protocolVersion": "2024-11-05",
+    "capabilities": {},
+    "clientInfo": { "name": "ExampleClient", "version": "1.0.0" }
+  }
+}
+-- server --
+{
+	"jsonrpc": "2.0",
+	"id": 1,
+	"result": {
+		"_meta": {},
+		"capabilities": {
+			"completions": {},
+			"logging": {},
+			"prompts": {
+				"listChanged": true
+			},
+			"resources": {
+				"listChanged": true
+			},
+			"tools": {
+				"listChanged": true
+			}
+		},
+		"protocolVersion": "2024-11-05",
+		"serverInfo": {
+			"name": "testServer",
+			"version": "v1.0.0"
+		}
+	}
+}