fix: Tape.__getitem__ ignores slice step; last_actions() breaks on n_added_steps=0

[mittalpk]

Two related bugs in tapeagents/core.py's Tape class, found via code review (no existing issue).

1. Tape.__getitem__ silently ignores a slice's step

return self.model_copy(update=dict(steps=self.steps[key.start : key.stop], metadata=TapeMetadata()))

Reconstructs the slice from start/stop only, dropping key.step entirely. tape[::-1], tape[::2], tape[1:5:2] etc. all silently return the wrong steps instead of applying the step or raising. Fixed by passing the slice object through directly (self.steps[key]), which Python already handles correctly.

2. last_actions() re-executes the entire tape history when n_added_steps == 0

def last_actions(tape: Tape) -> list[Action]:
    return [step for step in tape.steps[-tape.metadata.n_added_steps :] if isinstance(step, Action)]

n_added_steps defaults to 0 and __add__ legitimately produces 0 when zero new steps were added (see the existing test_add_empty_tape). Python's -0 == 0, so tape.steps[-0:] returns the entire list instead of an empty slice.

This matters because Environment.react()/areact() (environment.py, remote_environment.py) call last_actions(tape) to decide which of the agent's latest actions to execute against tools. With n_added_steps == 0, they'd silently re-execute every action ever taken in the tape's whole history and append duplicate observations, instead of doing nothing.

How did you test it

Added 4 tests to tests/test_core.py: test_getitem_slice_with_step_reversed, test_getitem_slice_with_step_stride, test_last_actions_zero_added_steps_returns_empty, test_last_actions_positive_n_added_steps_unchanged. Confirmed all 4 fail against the pre-fix code (git stash) and pass after.

uv run pytest tests/ --ignore-glob="tests/*/*" (the CI's make test-core target): 62 passed. uv run ruff check clean on both changed files.