Reject a malformed BASELINE_SYNC instead of ignoring it

[josephdviviano]

_apply_baseline_sync read the payload with .get and skipped the assignment when it returned None:

global_baseline = msg.message_data.get("global_baseline_log_reward")
if global_baseline is not None:
    self._global_baseline = float(global_baseline)

A BASELINE_SYNC with a missing key, a null value, or a non-finite value left the manager on its shard-local threshold and returned normally. The adopted value gates what every assigned worker sends, so that manager filters against a different cutoff from its siblings — the state the message exists to remove — and nothing in the run says so.

It now raises ValueError on a non-dict payload, on a missing or null global_baseline_log_reward, and on a non-finite one. A non-finite baseline keeps every trajectory or none.

testing/test_replay_buffer.py::test_manager_baseline_sync_ignores_missing_payload asserted the old behaviour, so it is replaced rather than added to. The new test is parametrized over all four malformed payloads and checks _global_baseline is untouched.

testing/test_replay_buffer.py and testing/test_dependency_free.py: 100 passed, 1 skipped. Each new case verified to fail against the previous implementation. black, isort and flake8 clean.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.64%. Comparing base (f39bfc9) to head (614d4bf).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #533      +/-   ##
==========================================
+ Coverage   75.62%   75.64%   +0.01%     
==========================================
  Files          62       62              
  Lines       11329    11335       +6     
  Branches     1670     1672       +2     
==========================================
+ Hits         8568     8574       +6     
  Misses       2234     2234              
  Partials      527      527              

Files with missing lines	Coverage Δ
src/gfn/containers/replay_buffer_manager.py	42.10% <100.00%> (+1.88%)	⬆️

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