[BUG] Slack Delivery Method Fails with JSON::ParserError on Non-JSON Response

[coatezy]

Bug Report

Describe the Bug:
The Slack delivery method throws an error when attempting to parse a plain text response from the Slack webhook endpoint as JSON.

When setting up a Slack notification, the limited documentation and accepted configuration led me to believe that the delivery method expected the incoming_webhook.url returned during the OAuth handshake.

After further investigation and checking the default URL in the class, I found that it actually calls the https://slack.com/api/chat.postMessage endpoint, which returns a JSON response. After setting the Authorization header and specifying the channel in the JSON body, I was able to deliver a message. However:

• It would be helpful if we could optionally use the webhook URL, as this approach requires less configuration.
• Alternatively, the documentation should clearly state that this delivery method is designed to work with the chat.postMessage endpoint.

Failed enqueuing Noticed::EventJob to Inline(default): JSON::ParserError (unexpected character: 'ok')
↳ (pry):4:in `<main>'
JSON::ParserError: unexpected character: 'ok'
from /Users/tomcoates/.rbenv/versions/3.3.4/lib/ruby/gems/3.3.0/gems/json-2.10.1/lib/json/common.rb:245:in `parse'

To Reproduce:

1. Configure notifier:

# frozen_string_literal: true

class TestNotifier < ApplicationNotifier
  deliver_by :slack do |config|
    config.url = -> { slack_webhook_url }
    config.json = -> do
      {
        text: "Hello, world"
      }
    end
  end

  private

  notification_methods do
    def slack_webhook_url
      # Set your webhook URL
      "https://hooks.slack.com/services/your/webhook/url"
    end
  end
end

2. Deliver notification:

TestNotifier.deliver(User.first)

Expected Behavior:
Requests should not raise an error unless config.raise_if_not_ok = true is set. The response should be correctly processed based on its content type.

Actual Behavior:
A JSON::ParserError is raised when the response is not JSON.

Screenshots (if applicable):

Environment:

• Noticed gem version: 2.5.1, 2.6.0
• Ruby version: 3.3.4
• Rails version: 8.0.1
• Operating System: macOS

Possible Fix:

• Only call JSON.parse if the response has Content-Type: application/json.
• Improve documentation to clarify that the delivery method is intended for chat.postMessage.

Checklist:

• I have searched for similar issues and couldn't find any.
• I have checked the documentation for relevant information.
• I have included all the required information.