feat: interactive messages — action handlers for buttons and menus

[jordanpartridge]

Summary

• InteractiveAction DTO wrapping Mattermost's interactive webhook payload with typed accessors (action ID, user, channel, post, context, type, trigger ID)
• InteractiveActionHandler abstract base class resolved from the container for constructor DI
• InteractiveActionRouter with route-style register(actionId, handler|closure) and dispatch() — mirrors SlashCommandRouter
• InteractiveActionResponse fluent builder supporting update() (replace original post text), props() (replace attachments), and ephemeral() (whisper to acting user)
• InteractiveActionController webhook endpoint auto-registered at mattermost/interactive via config
• Manager + Facade integration: Mattermost::interactive('action-id', Handler::class) and Mattermost::interactiveActionRouter()
• Config section mattermost.interactive with route and middleware options
• Full Pest test coverage (describe/it BDD) — DTO, response builder, router, and controller HTTP tests

Test plan

• vendor/bin/pint --test passes
• vendor/bin/pest --compact — 302 passed (all new Interactive tests green)
• vendor/bin/phpstan analyse — no errors at level 8
• vendor/bin/rector process --dry-run — no changes suggested

Closes #8

Demo: http://localhost:8065/jordan-partridge-enterprises-llc/pl/fwp7wfnq67bdmqkkp1jp3x874y

[lexi-chief-of-staff]

Blocking: Interactive webhooks use application/x-www-form-urlencoded with payload param holding JSON-encoded string — not direct JSON body. $request->all() gives ['payload' => '{...}']; DTO then fails as actionId() etc. expect flat payload. Real payload nests actions under top-level 'action' => [{ 'id' => '...', ... }], not $payload['action'] scalar.

Fix controller:

$payloadString = $request->input('payload');
$payload = json_decode($payloadString ?? '', true) ?? [];
if (json_last_error() !== JSON_ERROR_NONE) {
    return new JsonResponse(['ephemeral_text' => 'Invalid payload'], 400);
}
$action = InteractiveAction::fromPayload($payload);

Fix DTO accessors (e.g. actionId()): Extract from $payload['action'][0]['id'] ?? '' etc. Update docblocks (type() is top-level payload type like 'interactive_message', not element type).

Fix tests: Use $this->post(..., ['payload' => json_encode($fixture)]); update fixtures to nest action => [{'id' => '...'}]; drop bogus context['action'] expectation.

Mirrors slash commands well otherwise (which use flat form data [from: GitHubReadFile conduit-ui/mattermost:src/SlashCommands/SlashCommandController.php]). Full Pest coverage + Pint/Pest/PHPStan/Rector green is solid.