fix(core): avoid KeyError when batch_size is given without num_envs

[hobostay]

Problem

envpool.make(...) raises a KeyError: 'num_envs' whenever batch_size is supplied without an explicit num_envs.

In EnvRegistry._make_env_spec (envpool/registration.py), the batch_size validation indexed kwargs["num_envs"] directly:

if "batch_size" in kwargs:
    assert 0 <= kwargs["batch_size"] <= kwargs["num_envs"]   # KeyError

…while the two seed-normalization blocks immediately above already use the safe form kwargs.get("num_envs", 1). num_envs is never injected into kwargs — it only appears there if the user passes it — so the bare subscript crashes before gen_config is ever reached.

This is reachable through the documented public API:

• batch_size is a first-class make() argument (README / docs/content/python_interface.rst).
• batch_size=0 is a valid value meaning "default to num_envs" (core/env_spec.h turns batch_size == 0 into num_envs).

Reproduction

import envpool
envpool.make("Pong-v5", env_type="gymnasium", batch_size=0)
# KeyError: 'num_envs'   <- before this PR

Fix

Use the same default as the neighboring validation (one-line change, matches the existing kwargs.get("num_envs", 1) pattern):

if "batch_size" in kwargs:
    assert 0 <= kwargs["batch_size"] <= kwargs.get("num_envs", 1)

Behavior after the fix:

call	before	after
batch_size=0 (no num_envs)	KeyError ✗	OK (defaults to num_envs)
batch_size=1 (no num_envs)	KeyError ✗	OK
batch_size=2 (no num_envs, out of range)	KeyError ✗	AssertionError (correctly rejected)
num_envs=64, batch_size=16 (README usage)	OK	OK (unchanged)

The documented multi-env usage is unaffected; only the spurious KeyError is removed and out-of-range values are now rejected with a proper AssertionError.

Test

Added test_make_batch_size_without_num_envs to envpool/make_test.py, which fails (KeyError) before the fix and passes after.

Validation

make ruff py-format   # pass
make mypy             # changed files clean (under py311 target)