Add stateless MCP elicitation support

[pushpak1300]

Asking the user something mid-request used to need a live bidirectional session, and we don't have one — every request is handled on its own.

Elicitation in revision 2026-07-28 is stateless. The server answers with resultType: input_required, the client collects the answer, then retries the same call with it. Two plain requests.

So a handler can now ask for something:

$github = $request->ask('Your GitHub username', fn (JsonSchema $schema): array => [
    'name' => $schema->string()->required(),
]);

if ($github->declined()) {
    return Response::error('No problem, skipping.');
}

return Response::text("Hi {$github['name']}");

What $github is

An ElicitResponse wrapping the client's reply — an action, plus content matching the schema you asked for:

{"action": "accept", "content": {"name": "octocat"}}

$github->accepted();               // submitted, so content is populated
$github->declined();               // said no outright, offer an alternative
$github->cancelled();              // dismissed without choosing, fine to ask again later

$github['name'];                   // a field from content
$github->get('name', 'anonymous'); // with a fallback
$github->validate(['name' => 'required|string']);

It is read only, and reading content when the action is not accept throws.

Ask more than once and each answer arrives in turn:

$name = $request->ask('Your name', fn (JsonSchema $schema): array => [
    'name' => $schema->string()->required(),
]);

$colour = $request->ask('Favourite colour', fn (JsonSchema $schema): array => [
    'colour' => $schema->string()->required(),
]);

return Response::text("{$name['name']} likes {$colour['colour']}");

An ask() with no answer yet ends the request and asks the client. The handler re-runs from the top on the retry with the earlier answers already filled in, so $name is populated by the time the second round reaches $colour. Anything before an ask() runs on every round — keep it idempotent.

Works in tools, prompts, resources and generator tools. Reading a declined or cancelled response throws, so check accepted() or declined() first.

Not supported through tool search. A catalog tool reached via execute_tools that calls ask() fails with Tool [name] requested user input, which is not supported through tool search. — a batch has no way to replay the calls that already completed, so a round trip would re-run their side effects.

Testing:

ExampleServer::tool(SignUpTool::class)
    ->assertElicits('Your GitHub username')
    ->respond(['name' => 'octocat'])
    ->assertSee('Hi octocat');