fix: explain oversized system prompts

[w3lld1]

Summary

• detect oversized string system_prompt values before Linux rejects the CLI subprocess with a raw Argument list too long error
• include the measured prompt size, platform limit, and system_prompt={"type": "file", ...} workaround in the SDK error
• document the file-based system prompt form in ClaudeAgentOptions

Fixes #1096

Test plan

• uv run --extra dev ruff format src/claude_agent_sdk/_internal/transport/subprocess_cli.py src/claude_agent_sdk/types.py tests/test_transport.py
• uv run --extra dev ruff check src/claude_agent_sdk/_internal/transport/subprocess_cli.py src/claude_agent_sdk/types.py tests/test_transport.py
• uv run --extra dev mypy src/claude_agent_sdk/_internal/transport/subprocess_cli.py src/claude_agent_sdk/types.py
• uv run --extra dev pytest tests/test_transport.py -q

[tapheret2]

Review notes on “fix: explain oversized system prompts”

Touched files (3): src/claude_agent_sdk/_internal/transport/subprocess_cli.py, src/claude_agent_sdk/types.py, tests/test_transport.py
Diff size (approx): +75 / -1

Observations

• Nice that tests/fixtures are included with the change — that helps a lot for review confidence.

Questions / nits

• Does this need a changelog / release note entry for the target version?
• Any user-facing behavior change that should be called out in the PR body?

Thanks @w3lld1 for the contribution — left this as an independent review pass.