fix: update slack-go/slack to v0.17.3 to fix JSON unmarshal error

[ljagiello]

Summary

• Update slack-go/slack from v0.17.1 to v0.17.3
• Add test case to verify the fix

Problem

When Slack API returns errors as strings instead of objects, the library fails with:

json: cannot unmarshal string into Go struct field chatResponseFull.SlackResponse.errors of type map[string]interface{}

This happens because SlackResponseErrors.UnmarshalJSON in v0.17.1 doesn't handle string error values:

var raw map[string]interface{}
if err := json.Unmarshal(data, &raw); err != nil {
    return err   // fails on string errors like ["error_code"]
}

Root Cause

This is a known bug in slack-go/slack, tracked and fixed upstream:

• Issue: SlackResponse errors struct does not handle all errors slack-go/slack#1446
• Fix PR: fix: parse simple string based errors as part of the response slack-go/slack#1452
• Released in: v0.17.3

Fix

Simply update the dependency to v0.17.3 which includes the fallback for string errors:

// If we can't unmarshal as object, try as string (fallback case)
var stringError string
if stringErr := json.Unmarshal(data, &stringError); stringErr == nil {
    e.Message = &stringError
    return nil
}

Test plan

• Added TestSlackErrorStringUnmarshal to verify string errors are handled
• Added TestSlackErrorObjectUnmarshal to verify object errors still work
• All tests pass with go test ./...