Integration: REST APIs and platform events

export INSTANCE_URL='https://yourdomain.my.salesforce.com'
export TOKEN='paste-the-access-token-here'

# Which API versions does this org support? A quick test that the token works.
curl "$INSTANCE_URL/services/data/" -H "Authorization: Bearer $TOKEN"

curl -G "$INSTANCE_URL/services/data/v62.0/query" \
  --data-urlencode "q=SELECT Id, Name, Industry FROM Account LIMIT 3" \
  -H "Authorization: Bearer $TOKEN"

curl -X POST "$INSTANCE_URL/services/data/v62.0/sobjects/Account" \
  -H "Authorization: Bearer $TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"Name": "Lakeview Foods", "Industry": "Retail"}'

curl -i -X PATCH "$INSTANCE_URL/services/data/v62.0/sobjects/Account/001XXXXXXXXXXXXXXX" \
  -H "Authorization: Bearer $TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"Phone": "080-5550100"}'

@RestResource(urlMapping='/accounts/*')
global with sharing class AccountApi {

    // GET /services/apexrest/accounts/?industry=Retail
    @HttpGet
    global static List<Account> getByIndustry() {
        String industry = RestContext.request.params.get('industry');
        if (String.isBlank(industry)) {
            RestContext.response.statusCode = 400;
            return new List<Account>();
        }
        return [
            SELECT Id, Name, Industry
            FROM Account
            WHERE Industry = :industry
            LIMIT 50
        ];
    }

    // POST /services/apexrest/accounts/ with a JSON body {"name": "...", "industry": "..."}
    @HttpPost
    global static Id createAccount(String name, String industry) {
        Account acc = new Account(Name = name, Industry = industry);
        insert acc;
        RestContext.response.statusCode = 201;
        return acc.Id;
    }
}

curl -G "$INSTANCE_URL/services/apexrest/accounts/" \
  --data-urlencode "industry=Retail" \
  -H "Authorization: Bearer $TOKEN"

curl -X POST "$INSTANCE_URL/services/apexrest/accounts/" \
  -H "Authorization: Bearer $TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"name": "Green Leaf Traders", "industry": "Retail"}'

@IsTest
private class AccountApiTest {

    @IsTest
    static void getByIndustryReturnsMatches() {
        insert new Account(Name = 'Test Retail', Industry = 'Retail');

        RestRequest req = new RestRequest();
        req.requestURI = '/services/apexrest/accounts/';
        req.addParameter('industry', 'Retail');
        RestContext.request = req;
        RestContext.response = new RestResponse();

        List<Account> result = AccountApi.getByIndustry();

        System.assertEquals(1, result.size());
        System.assertEquals('Test Retail', result[0].Name);
    }

    @IsTest
    static void missingIndustryGives400() {
        RestContext.request = new RestRequest();
        RestContext.response = new RestResponse();

        List<Account> result = AccountApi.getByIndustry();

        System.assertEquals(400, RestContext.response.statusCode);
        System.assertEquals(0, result.size());
    }
}

public with sharing class WeatherService {

    public class WeatherException extends Exception {}

    public static Decimal getTempC(String city) {
        HttpRequest req = new HttpRequest();
        req.setEndpoint('callout:Weather_API/v1/current?city=' + EncodingUtil.urlEncode(city, 'UTF-8'));
        req.setMethod('GET');
        req.setTimeout(10000); // milliseconds

        HttpResponse res = new Http().send(req);
        if (res.getStatusCode() != 200) {
            throw new WeatherException('Weather API returned ' + res.getStatusCode());
        }
        Map<String, Object> data = (Map<String, Object>) JSON.deserializeUntyped(res.getBody());
        return (Decimal) data.get('tempC');
    }
}

@IsTest
private class WeatherServiceTest {

    private class MockOk implements HttpCalloutMock {
        public HttpResponse respond(HttpRequest req) {
            HttpResponse res = new HttpResponse();
            res.setStatusCode(200);
            res.setBody('{"city":"Bengaluru","tempC":27.5}');
            return res;
        }
    }

    private class MockFail implements HttpCalloutMock {
        public HttpResponse respond(HttpRequest req) {
            HttpResponse res = new HttpResponse();
            res.setStatusCode(500);
            res.setBody('error');
            return res;
        }
    }

    @IsTest
    static void parsesTemperature() {
        Test.setMock(HttpCalloutMock.class, new MockOk());
        Test.startTest();
        Decimal t = WeatherService.getTempC('Bengaluru');
        Test.stopTest();
        System.assertEquals(27.5, t);
    }

    @IsTest
    static void throwsOnServerError() {
        Test.setMock(HttpCalloutMock.class, new MockFail());
        try {
            WeatherService.getTempC('Bengaluru');
            System.assert(false, 'Expected an exception');
        } catch (WeatherService.WeatherException e) {
            System.assert(e.getMessage().contains('500'));
        }
    }
}

List<Order_Ready__e> events = new List<Order_Ready__e>{
    new Order_Ready__e(Order_Number__c = 'BB-1042', Customer_Name__c = 'Asha'),
    new Order_Ready__e(Order_Number__c = 'BB-1043', Customer_Name__c = 'Ravi')
};

List<Database.SaveResult> results = EventBus.publish(events);
for (Database.SaveResult sr : results) {
    if (sr.isSuccess()) {
        System.debug('Published');
    } else {
        System.debug('Failed: ' + sr.getErrors()[0].getMessage());
    }
}

trigger OrderReadyTrigger on Order_Ready__e (after insert) {
    List<Task> tasks = new List<Task>();
    for (Order_Ready__e evt : Trigger.new) {
        tasks.add(new Task(
            Subject = 'Order ' + evt.Order_Number__c + ' is ready for ' + evt.Customer_Name__c,
            Priority = 'Normal',
            Status = 'Not Started'
        ));
    }
    insert tasks;
}

@IsTest
private class OrderReadyTriggerTest {
    @IsTest
    static void createsTaskForEachEvent() {
        Test.startTest();
        EventBus.publish(new Order_Ready__e(Order_Number__c = 'BB-1', Customer_Name__c = 'Test'));
        Test.stopTest();

        List<Task> tasks = [SELECT Subject FROM Task WHERE Subject LIKE 'Order BB-1%'];
        System.assertEquals(1, tasks.size());
    }
}

I call the Salesforce REST API with curl and get this response:

[paste the status code and body]

My request was:

[paste the curl command, with the token replaced by XXXX]

Explain in plain English what the error means, list the 3 most likely causes in order, and tell me how to check each one in Setup.