mcp: handle bad or missing params (#210)

Audit all cases where params must not be null, and enforce this using
methodInfo via a new methodFlags bitfield that parameterizes method
requirements. Write extensive conformance tests catching all the
(server-side) crashes that were possible.

We should go further and validate schema against the spec, but that is
more indirect, more complicated, and potentially slower. For now we
adopt this more explicit approach.

Still TODO in a subsequent CL: verify the client side of this with
client conformance tests.

Additionally, improve some error messages that were redundant or leaked
internal implementation details.

Fixes #195

mcp/testdata/conformance/server/resources.txtar

@@ -2,6 +2,9 @@ Check behavior of a server with just resources.
 
 Fixed bugs:
 - A resource result holds a slice of contents, not just one.
+- No params in 'resource/read' should not panic.
+- No params in 'resources/subscribe' should not panic.
+- No params in 'resources/unsubscribe' should not panic.
 
 -- resources --
 info
@@ -39,6 +42,8 @@ info.txt
 	"roots": []
   }
 }
+{ "jsonrpc": "2.0", "id": 4, "method": "resources/read" }
+{ "jsonrpc": "2.0", "id": 5, "method": "resources/subscribe" }
 -- server --
 {
 	"jsonrpc": "2.0",
@@ -107,3 +112,19 @@ info.txt
 		]
 	}
 }
+{
+	"jsonrpc": "2.0",
+	"id": 4,
+	"error": {
+		"code": -32600,
+		"message": "invalid request: missing required \"params\""
+	}
+}
+{
+	"jsonrpc": "2.0",
+	"id": 5,
+	"error": {
+		"code": -32600,
+		"message": "invalid request: missing required \"params\""
+	}
+}