Decouple Trajectory Store protocols and store backends from Tunix-specific abstractions (TunixTrajectoryMetadata, TunixTrajectory, TunixAgentStep, and TunixEnvStep):

[copybara-service]

Decouple Trajectory Store protocols and store backends from Tunix-specific abstractions (TunixTrajectoryMetadata, TunixTrajectory, TunixAgentStep, and TunixEnvStep):

• Parameterize TrajectoryReader, TrajectoryWriter, and TrajectoryStore using TypeVar('T', bound=TrajectoryMetadata) and TypeVar('TrajT', bound=TrajectoryMetadata).
• Add a polymorphic create_trajectory factory method on TrajectoryMetadata and TunixTrajectoryMetadata to construct full Trajectory instances polymorphically.
• Remove Tunix-specific imports, isinstance branches, and static type ignores from InMemoryTrajectoryStore and FileTrajectoryStore.
• In FileTrajectoryStore, load step data directly as dictionaries and pass them to the trajectory factory, allowing Pydantic's recursive field validation to automatically instantiate the concrete step models (e.g., TunixAgentStep vs. TunixEnvStep) without requiring manual step type configurations.
• Add test cases in in_memory_store_test.py and file_store_test.py covering step_id=0 Tunix trajectories and custom TrajectoryMetadata/Trajectory subclasses.