Drop slack/web-api to replace axios with fetch

[Zaimwa9]

Context
Axios has triggered multiple CVEs (critical and high). We inherit it as a transitive dependency from @slack/web-api.
We can drop it by replacing the library with direct fetch calls

Steps to replace

1. Rewrite api/slack-client.js to replace WebClient.chat.postMessage with one fetch call.
2. Rewrite e2e/slack-e2e-reporter.ts to replace postMessage (one fetch) and files.uploadV2 (3-step flow: get upload URL → upload bytes → complete upload).
3. Drop the dep, remove @slack/web-api from package.json and the axios override