fireworks: retain root client refs to prevent aiohttp session leak

`ChatFireworks.validate_environment` extracted `.chat.completions` from
freshly created `Fireworks` / `AsyncFireworks` instances and immediately
discarded the parents.  Starting with `fireworks-ai>=0.19`, those parents
eagerly create an `aiohttp.ClientSession` when an async event loop is
running.  Orphaning the parents means the sessions are never closed,
producing "Unclosed client session" warnings at interpreter shutdown.

Store parent instances as `root_client` / `root_async_client` (matching
the `langchain-openai` pattern) so transports stay reachable and are
cleaned up deterministically.

Adds a regression test that constructs `ChatFireworks` inside an async
context and asserts no `ResourceWarning` about unclosed sessions fires.