Migrate R write_stdin cases to external runner

[t-kalinowski]

Summary

Move a cohesive R write_stdin public-behavior cluster from Rust integration tests into the external Python public API runner.

This keeps the external runner migration moving by verifying the real mcp-repl binary over MCP stdio while removing duplicate Rust coverage for the same public scenarios.

User Facing Changes

• No runtime behavior changes are expected.
• Public API coverage now exercises more R write_stdin behavior through the same external runner used in CI.

Internal Changes

• Added external runner cases for multiple calls, timeout polling, error recovery, huge echo-only input, and files-mode echo trimming.
• Removed matching Rust integration tests and now-unused helper code from tests/write_stdin_batch.rs.
• Stabilized two Windows prepared-launch sandbox tests by using the existing guarded temp-root helper for session temp setup.
• Updated the active public API runner plan with the migrated cluster.