feat(pty): share single Docker container across terminal tabs

[veeso]

Summary

• Introduces SharedContainer (Arc-wrapped) that creates a single Docker container at app startup with sleep infinity as entrypoint
• Each terminal tab spawns its own bash process via docker exec instead of creating a new container
• Container is automatically stopped and removed when the last tab (last Arc reference) is dropped
• Adds PtyProviderType enum to separate config-time provider selection from runtime PtyProvider (which carries the Arc<SharedContainer>)

Test plan

• cargo clippy --all-features -- -D warnings passes
• cargo clippy -- -D warnings passes (without feature)
• cargo test — 381 tests pass
• Manual: cargo run --features pty-test_container -- --use-pty-test-container, open multiple tabs with Cmd+T, verify all share one container (docker ps shows single infraware_* container)
• Manual: close tabs one by one, verify container removed only after last tab closed