Add tests for agent parameter and usingListModels()

[Copilot]

Tests were missing for two features added in the SDK sync: the agent parameter on session config types and the usingListModels() fluent method on Client.

SessionConfig / ResumeSessionConfig — agent field

• Expanded "all fields" fromArray / toArray tests to include agent
• Updated "minimal fields" tests to assert agent is null
• Added dedicated tests: constructor, toArray serialisation, fromArray parsing, null-exclusion from toArray

Client — usingListModels()

• Fluent interface returns $client (no CLI connection required)
• listModels() invokes the handler and returns ModelInfo[]
• usingListModels(null) accepted without error
• Empty handler returns []

$client = new Client;
$models = $client->usingListModels(fn () => [
    ['id' => 'my-model', 'name' => 'My Model', 'capabilities' => [...]],
])->listModels();

// $models[0] is a ModelInfo instance — no CLI connection needed

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