Add SystemPromptFile support for --system-prompt-file CLI flag

[ronmrdechai]

I'm trying to pass a really big system prompt to Claude and it fails on an ARGMAX error. This change exposes --system-prompt-file to the SDK.

Summary

• Add SystemPromptFile TypedDict with type: "file" and path fields to types.py
• Add SystemPromptFile to the system_prompt union type on ClaudeAgentOptions
• Handle the new type in subprocess_cli.py by passing --system-prompt-file <path> to the CLI
• Add test coverage in both test_types.py and test_transport.py

Test plan

• test_claude_code_options_with_system_prompt_file — verifies ClaudeAgentOptions accepts and stores a SystemPromptFile dict
• test_build_command_with_system_prompt_file — verifies the command builder emits --system-prompt-file with the path, and does not emit --system-prompt or --append-system-prompt
• All existing tests in test_types.py and test_transport.py continue to pass