fix: align strict validation error handling with PR modelcontextprotocol#1044 pattern

Change strict validation errors to return CallToolResult with isError: true
instead of throwing protocol-level errors, aligning with the pattern established
in PR modelcontextprotocol#1044 (commit 7387c44) where all validation errors return error responses
rather than throwing.

This ensures consistent error handling across all validation scenarios and
enables LLMs to see validation errors and attempt self-correction, as specified
in the MCP protocol specification.

Changes:
- Remove conditional throw for strict validation input errors
- Update test expectations to check for error responses instead of thrown errors
- Keep strictInputSchemaValidation field in RegisteredTool for future use

The strictInputSchemaValidation field is retained as it still serves to apply
.strict() to the Zod schema, rejecting unknown parameters. The difference is
now the rejection returns an error response visible to LLMs rather than a
protocol-level error.

Changes linted and tested.

<em>🤖 Created with Claude Code via Airchat</em>

Author: Adam Bloomston

src/server/mcp.test.ts

@@ -4228,17 +4228,24 @@ describe('elicitInput()', () => {
 
         await Promise.all([client.connect(clientTransport), mcpServer.server.connect(serverTransport)]);
 
-        await expect(
-            client.request(
-                {
-                    method: 'tools/call',
-                    params: {
-                        name: 'test-strict',
-                        arguments: { username: 'test', itemcount: 42 }
-                    }
-                },
-                CallToolResultSchema
-            )
-        ).rejects.toThrow('Invalid arguments');
+        const result = await client.request(
+            {
+                method: 'tools/call',
+                params: {
+                    name: 'test-strict',
+                    arguments: { username: 'test', itemcount: 42 }
+                }
+            },
+            CallToolResultSchema
+        );
+
+        expect(result.isError).toBe(true);
+        expect(result.content).toEqual(
+            expect.arrayContaining([
+                expect.objectContaining({
+                    text: expect.stringContaining('Input validation error')
+                })
+            ])
+        );
     });
 });

src/server/mcp.ts

@@ -174,15 +174,6 @@ export class McpServer {
                     }
                 }
             } catch (error) {
-                if (
-                    tool &&
-                    tool.strictInputSchemaValidation &&
-                    error instanceof McpError &&
-                    error.code === ErrorCode.InvalidParams &&
-                    error.message.includes('Input validation error')
-                ) {
-                    throw error;
-                }
                 return this.createToolError(error instanceof Error ? error.message : String(error));
             }