fix: reject NIDs that are not issues or do not exist

[mglaman]

What changed

Client::getNode() now throws when the NID is not an issue or does not exist. Every issue:* and mr:* command that resolves a NID surfaces the message and exits non-zero instead of printing empty fields.

Closes #367.

Why

Drupal.org serves every node type from node/{nid}.json, and getNode() mapped all of them to IssueNode:

$ drupalorg issue:get-fork 3000001   # a project_release
GitLab path: issue/-3000001

Unknown NIDs are worse: Drupal.org answers with HTTP 200 and {"comments":[],"body":[],...}, so issue:show 999999999 printed a blank issue with exit code 0.

How

$type = $data->type ?? null;
if (!is_string($type)) {
    throw new \RuntimeException(sprintf('Node %s was not found on Drupal.org.', $nid));
}
if ($type !== 'project_issue') {
    throw new \RuntimeException(sprintf('Node %s is a %s, not an issue.', $nid, $type));
}

The check lives in Client rather than IssueNode::fromStdClass() because the entity is intentionally lenient for partial data and the API response is the boundary where type is meaningful.

tests/src/ClientTest.php is new. Client builds its own Guzzle instance, so the test subclasses it and swaps $this->client for a MockHandler-backed one; no change to the public constructor.

Testing

• vendor/bin/phpcs src tests, vendor/bin/phpstan analyse src, vendor/bin/phpunit (167 tests) all pass.
• Live:
  • issue:show 3000001 → Node 3000001 is a project_release, not an issue.
  • issue:get-fork 3060 → Node 3060 is a project_core, not an issue.
  • mr:list 999999999 → Node 999999999 was not found on Drupal.org.
  • issue:show 3383637 → unchanged

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