Remove on_list_models constructor option from Client; keep only usingListModels()

[Copilot]

Client::__construct() accepted an on_list_models option, but there is no forceClient() on CopilotManager to inject a custom-constructed client, making this option unreachable via the public API. The only viable API is the fluent usingListModels() method.

Changes

• src/Client.php: Remove on_list_models extraction from $options in both the TCP and stdio constructor branches. Update listModels() docblock accordingly.
• docs/jp/custom-providers.md: Remove the on_list_models / Copilot::forceClient() example entirely; retain only the usingListModels() usage.

Only supported pattern

$models = Copilot::client()->usingListModels(function (): array {
    return [
        ['id' => 'my-model-1', 'name' => 'My Model 1', 'version' => '1.0'],
    ];
})->listModels();

// Remove handler and fall back to CLI server
$models = Copilot::client()->usingListModels(null)->listModels();

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