Builder::build() silently skips configured file-based discovery when symfony/finder is missing — should fail loudly

Bắt đầu bằng cách xác định Builder::build() và phần xử lý setDiscovery() được mô tả trong issue, sau đó so sánh nhánh thiếu Finder của nó với Discoverer::__construct(). Thêm hoặc cập nhật một test tập trung cho một đường dẫn discovery đã được cấu hình mà không có symfony/finder. Hoàn thành khi build() ném RuntimeException hiện có, có thể hành động được, thay vì ghi log cảnh báo và tạo ra một danh sách công cụ rỗng.

Calling setDiscovery() is an explicit declaration that the server's tools come from attribute discovery. But since symfony/finder is only a suggested dependency, a host application can easily end up without it — and when that happens, build() does this:

if (null !== $this->discoveryBasePath) {
    if (null !== $this->discoverer || class_exists(Finder::class)) {
        // ... discovery runs
    } else {
        $logger->warning('File-based discovery requires symfony/finder...');
    }
}

The result is the worst failure mode available: the server builds successfully, initialize succeeds, and tools/list returns an empty array. The operator sees a healthy server; the symptom surfaces far from the cause, as confused MCP clients with no tools. The only breadcrumb is a single warning log line.

This also contradicts the SDK's own Discoverer::__construct(), which already throws RuntimeException('File-based discovery requires symfony/finder. ...') for exactly this situation — the builder's class_exists pre-check just routes around that guard, downgrading a configured-but-impossible feature from an error to a whisper.

Proposal: when discoveryBasePath is set, no custom discoverer was supplied, and Finder is unavailable, build() should throw (the Discoverer's existing message is perfect) instead of warning-and-skipping. This costs nothing for explicit-registration users, client-only users, or anyone without setDiscovery() — it only converts a silent production mystery into an immediate, actionable boot error for people who asked for discovery and can't have it.

Observed on v0.7.0. Context: we hit this failure mode while integrating the SDK into a Symfony bundle (pimcore/data-hub-simple-rest#312) and worked around it by requiring symfony/finder in the bundle directly — which remains the right consumer-side fix, but doesn't help the next integrator who doesn't know about the silent path.