Fixes #76670.\n\nSummary:\n- Collect the Mattermost bot token and server URL as separate wizard patches so validation does not run before the URL is entered.\n- Preserve non-interactive Mattermost setup validation for explicit --bot-token + --http-url flows.\n- Add a regression test and changelog entry.\n\nVerification:\n- Reporter manually verified setup against a real Mattermost server.\n- pnpm test extensions/mattermost\n- pnpm tsgo:extensions\n- pnpm tsgo:extensions:test\n- pnpm exec oxfmt --check --threads=1 extensions/mattermost/src/setup-core.ts extensions/mattermost/src/setup-surface.ts extensions/mattermost/src/setup.test.ts\n- git diff --check upstream/main...HEAD
