feat: return Errors field in SlackErrorResponse

[nlopes]

There are a few methods in the Slack API that return an Errors field.

We've historically not captured this. It seems there are 4:

• https://slack.com/api/conversations.invite
• https://slack.com/api/apps.manifest.create
• dialog submission
  This one is a bit odd, I only found it in the java docs, not sure where it is in the API docs. In any case, we don't support it in the library (I think)
• https://slack.com/api/admin.workflows.collaborators.add

We do not support the last two in the library therefore I didn't even add at this stage.

This PR adds support for the first two in the list.

Fixes #1433.

[Copilot]

Pull Request Overview

This PR adds support for capturing and returning the errors field in Slack API responses for the /apps.manifest.create and /conversations.invite methods.

• Introduces two new error structs (AppsManifestCreateResponseError, ConversationsInviteResponseError) and a union wrapper (SlackResponseErrors) with custom JSON (un)marshaling.
• Extends SlackResponse and SlackErrorResponse to carry a slice of these errors.
• Provides end-to-end tests and updates the example for handling conversation-invite errors.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
misc.go	Added new error types, union type, and updated response structs
misc_test.go	Added tests for marshaling/unmarshaling and top‐level errors
examples/conversations_invite/conversations_invite.go	Updated example to iterate over the new Errors slice

Comments suppressed due to low confidence (3)

misc.go:45

• [nitpick] The type name SlackResponseErrors is plural but represents a single error union. Consider renaming it to SlackResponseError for clarity, and adjust the slice field to []SlackResponseError.

type SlackResponseErrors struct {

misc_test.go:285

• The test only checks for non-nil/len-zero Errors but doesn’t verify the actual contents of the parsed error structs. Add assertions to compare the fields (e.g. Pointer, Message, User) against the expected values.

if len(tt.expected.Errors) == 0 {

misc.go:126

• [nitpick] The new Errors field on SlackErrorResponse lacks a comment explaining its purpose. Adding a doc comment will help users understand how to consume this slice of error unions.

Errors           []SlackResponseErrors