[codex] Expose runtime image handle across SDKs

[DorianZheng]

What changed

• expose runtime-scoped image handles in Python async/sync, Node, Go, and C
• add shared FFI image handle creation, pull, list, and free operations with JSON metadata serialization
• add SDK docs/examples and end-to-end tests for image pull/list plus unsupported REST paths

Why

The Rust runtime already supports image operations, but the public SDKs did not expose them. This change brings pull and list parity to the SDKs without leaking the internal Rust image object model.

Impact

• SDK users can call runtime.images.pull(...) and runtime.images.list()
• Go gets a new Images handle API
• C gets boxlite_runtime_images, boxlite_image_pull, boxlite_image_list, and boxlite_image_free
• REST-backed SDK runtimes continue to surface image operations as unsupported

Validation

• cargo test -p boxlite-ffi
• make test:all:python
• make test:all:node
• make test:all:c
• make test:all:go
• cargo fmt --check
• cargo clippy -p boxlite-ffi -p boxlite-c -p boxlite-python -p boxlite-node --tests -- -D warnings
• ruff check . && ruff format --check . in sdks/python
• npm run format:check in sdks/node