feat: first-class structured output support in AiClient

[roboshyim]

What

Exposes a clean, typed, provider-agnostic structured-output API on the AiClient facade (P0 roadmap item #4). The ChatRequest value object and Capability::StructuredOutput already existed — this wires them into a usable public API so features (property suggestion, SEO JSON, review summaries) and external plugin devs can request validated JSON without hand-rolling schema plumbing.

Public API

$response = $aiClient->chatStructured($messages, $schema, $options);
if ($response->isValid()) {
    $data = $response->data;          // decoded array
    $title = $response->get('title'); // convenience accessor
} else {
    $error = $response->error;        // why it failed
}
$raw = $response->raw; // underlying ChatResponse (usage, finishReason)

Implementation

• AiClient::chatStructured(array $messages, array $schema, array $options = []): StructuredResponse — builds a ChatRequest with responseSchema, routes via ModelResolver to a StructuredOutput-capable provider, parses the result.
• StructuredResponse (src/Response/) — typed wrapper: ->data, ->raw, ->error, ->isValid(), ->get(key, default).
• StructuredOutputParser (src/Client/) — robust extraction: strips ```json markdown fences, pulls the first valid JSON object/array from prose, graceful failure (returns invalid response, never fatals).

Tests

tests/Unit/Client/StructuredOutputTest.php (286 lines): schema passthrough to ChatRequest, clean JSON, fenced JSON, invalid JSON → invalid response, facade routing. Suite green in container (Shopware 6.7.2.2 / PHP 8.3).

Complements the Anthropic provider PR (#3), which implements the tool-use side of structured output.