mcp: return input validation errors as tool results, not JSON-RPC errors (#863)

This implements [SEP-1303](https://github.com/modelcontextprotocol/modelcontextprotocol/issues/1303).

Input validation errors (missing required fields, wrong types) during
tool calls
are now returned as `CallToolResult` with `IsError=true` instead of
JSON-RPC
`-32602` errors. This allows LLMs to see the validation error and
self-correct,
  per the MCP spec.
The existing pattern for handler-returned errors (lines 347-350 in
server.go)
already converts regular errors to tool results with `IsError=true`.
This change
applies the same pattern to argument schema validation and unmarshal
errors,
  resolving the `TODO(#450)` in the code.                   

Thanks to @iwyrkore for filing the issue and proposing the approach, and
  @findleyr for the cross-SDK analysis in the discussion.
  Fixes #450

Co-authored-by: Maciej Kisiel <mkisiel@google.com>

mcp/testdata/conformance/server/tools.txtar

@@ -222,9 +222,14 @@ inc
 {
 	"jsonrpc": "2.0",
 	"id": 8,
-	"error": {
-		"code": -32602,
-		"message": "invalid params: validating \"arguments\": validating root: required: missing properties: [\"In\"]"
+	"result": {
+		"content": [
+			{
+				"type": "text",
+				"text": "validating \"arguments\": validating root: required: missing properties: [\"In\"]"
+			}
+		],
+		"isError": true
 	}
 }
 {
@@ -245,9 +250,14 @@ inc
 {
 	"jsonrpc": "2.0",
 	"id": 10,
-	"error": {
-		"code": -32602,
-		"message": "invalid params: validating \"arguments\": validating root: required: missing properties: [\"Name\"]"
+	"result": {
+		"content": [
+			{
+				"type": "text",
+				"text": "validating \"arguments\": validating root: required: missing properties: [\"Name\"]"
+			}
+		],
+		"isError": true
 	}
 }
 {