Stabilize Windows integration test harness

[t-kalinowski]

Summary

• Resolve extensionless integration-test binary paths to .exe on Windows.
• Spawn test server processes with plain pager environment defaults: PAGER=cat and MANPAGER=cat.
• Split the reticulate py_help() regression into separate budgets for R startup, reticulate/Python initialization, and the final short help-render call.

Public-facing changes

• None.

Internal-only changes

• Windows integration test harness path resolution.
• Shared Rust and Python integration test launchers now default external pager environment variables to plain output.
• Reticulate help test timing now isolates startup and Python initialization from the behavior under assertion.

Testing

• cargo test --test r_startup test_server_spawns_with_plain_pager_env -- --nocapture
• cargo test --test reticulate_py_help -- --nocapture
• python3 -m unittest tests/test_run_integration_tests.py
• python3 tests/run_integration_tests.py --binary target/debug/mcp-repl
• cargo check
• cargo build
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test
• cargo +nightly fmt