fix(config): reject non-positive explorer_sync_interval / trainer_sync_interval

[Linxiushen]

Problem

SynchronizerConfigValidator asserts that synchronizer.sync_interval is positive, but the two per-role overrides that default to it, explorer_sync_interval and trainer_sync_interval, have no range check. Setting either to 0 passes that validator and then crashes further down the validator chain:

File "trinity/common/config_validator.py", line 895, in validate
    if config.explorer.eval_interval % config.synchronizer.explorer_sync_interval != 0:
ZeroDivisionError: integer modulo by zero

Reachability, stated plainly: the two fields are not mentioned in the docs or in any example config (they are only described in a comment in config.py), so a user has to know about them from the dataclass. Once used, 0 is an easy mistake for "sync every step". The consequence is a confusing traceback at config time instead of a clear validation error; there is no wrong numeric result.

Fix

Add the same positivity assertions the validator already uses for sync_interval, placed right after the two set_if_none calls so that inherited values are checked too:

assert config.synchronizer.explorer_sync_interval > 0, "`explorer_sync_interval` must be positive."
assert config.synchronizer.trainer_sync_interval > 0, "`trainer_sync_interval` must be positive."

Any configuration with positive values is unaffected. I ran all 74 YAML files under examples/ and tests/template through the validator chain before and after the change: outcomes and the resolved interval values are identical (none of them set these fields).

Test

test_sync_interval_overrides_must_be_positive in tests/common/config_test.py, one subtest per field. Without the fix both subtests fail with the ZeroDivisionError above; with it they pass with the expected AssertionError.

Placed next to test_multinode_vllm_requires_full_node_occupancy rather than at the end of the class, to avoid a textual conflict with the test that #613 adds at the end.

pre-commit pins (black 23.7.0, isort 5.12.0, flake8 6.1.0) pass on both files.

---

This fix was developed with AI assistance (Claude); the change and the test were reviewed and verified locally before submission.