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/tools.txtar

@@ -4,6 +4,7 @@ Fixed bugs:
 - "tools/list" can have missing params
 - "_meta" should not be nil
 - empty resource or prompts should not be returned as 'null'
+- the server should not crash when params are passed to tools/call
 
 -- tools --
 greet
@@ -22,6 +23,7 @@ greet
 { "jsonrpc": "2.0", "id": 2, "method": "tools/list" }
 { "jsonrpc": "2.0", "id": 3, "method": "resources/list" }
 { "jsonrpc": "2.0", "id": 4, "method": "prompts/list" }
+{ "jsonrpc": "2.0", "id": 5, "method": "tools/call" }
 -- server --
 {
 	"jsonrpc": "2.0",
@@ -81,3 +83,11 @@ greet
 		"prompts": []
 	}
 }
+{
+	"jsonrpc": "2.0",
+	"id": 5,
+	"error": {
+		"code": -32600,
+		"message": "invalid request: missing required \"params\""
+	}
+}