fix(core): extract usage metadata from serialized tracer message outputs (#35526)

Fixes missing `run.metadata.usage_metadata` population in
`LangChainTracer` for real LLM/chat traces following #34414

- Fix extraction to read usage from serialized tracer message shape:
`outputs.generations[*][*].message.kwargs.usage_metadata`
- Remove non-serialized direct message shape handling
(`message.usage_metadata`) from extractor to match real tracer output
path
- Clarify tracer docstrings around chat callback naming
(`on_chat_model_start` + shared `on_llm_end`) to reduce ambiguity

## Why

#34414 introduced usage duplication into `run.metadata.usage_metadata`,
but the extractor read `message.usage_metadata`.

In real tracer flow, messages are serialized with `dumpd(...)` during
run completion, so usage metadata lives under
`message.kwargs.usage_metadata`. Because of this mismatch, duplication
did not trigger in real traces.