[Tinker API] Keep forwarded sample futures in memory

[yapdianang]

Summary

• Keep forwarded sample futures in process memory.
• Return JSON and protobuf sample results through the same future API.
• Remove a completed result in constant time when the client retrieves it.
• Keep training futures in the database.

Depends on #2043.

Before / After

Path	Before	After
Forwarded sample submit	Create an external future row in SQLite	Create an in-memory future ID
Forwarded sample completion	Write the sample result to SQLite	Complete the in-memory future
Forwarded sample retrieval	Read and then clean up the database row	Pop the result from memory in constant time
Training operations	Database-backed	Database-backed

This removes sample-result database traffic from the high-concurrency rollout path. The store has a fixed capacity and rejects new futures when full.

Testing

uv run --extra dev --extra jax --extra tinker pytest \
  tests/tinker/test_future_waiting.py \
  tests/tinker/test_in_memory_future_store.py \
  tests/tinker/test_proto_serialization.py \
  tests/tinker/skyrl_train/test_async_sample_routing.py \
  tests/tinker/test_api.py
uv run --extra dev pre-commit run --all-files

The focused API tests pass: 45 passed and 4 skipped. All pre-commit hooks pass.