Add setModel() convenience method to Session

[Copilot]

Tracks official SDK PR #621. Adds setModel() to Session as a thin convenience wrapper over $this->rpc()->model()->switchTo().

Changes

• src/Session.php — adds setModel(string $modelId): void
• src/Contracts/CopilotSession.php — adds method to interface with @throws JsonRpcException
• src/Testing/FakeSession.php — no-op implementation to satisfy interface

Usage

$session->setModel('claude-sonnet-4');

// Equivalent to:
$session->rpc()->model()->switchTo(new SessionModelSwitchToParams(modelId: 'claude-sonnet-4'));

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