refactor(llm): rename Usage.native to providerMetadata

Aligns the escape-hatch field name with `LLMEvent.providerMetadata` used
elsewhere in this package (and with AI SDK / pydantic-ai / LangChain
conventions for the same idea). Two parallel escape hatches having
different names was a wart.

The raw payload is now wrapped under the provider key — `{ openai: ... }`,
`{ anthropic: ... }`, `{ google: ... }`, `{ bedrock: ... }` — using the
existing `ProviderMetadata = Record<string, Record<string, unknown>>`
schema rather than a flat record. Same shape as
`LLMEvent.providerMetadata`, so consumers downstream can read both with
the same code.

Anthropic's `mergeUsage` merges the per-provider sub-record across
`message_start` and `message_delta` instead of spreading at the top level.