fix: Ensure orphaned sub-registries can resolve names after pickling

[feussy]

Summary

Fixes get_registered_names() returning [] for models whose _registrations chain passes through a deserialized RootModelRegistry.

get_registered_names() terminates the chain with [""] only when self is ModelRegistry.root() — an identity check against the process-level singleton _REGISTRY_ROOT. When a model is pickled (e.g. sent to a Ray worker via ray.remote) and unpickled, the RootModelRegistry stored in the _registrations chain is deserialized as a new Python object. The is identity check fails, the chain falls through to [], and all model names are silently lost. Any evaluator logic keyed by model name is bypassed without error.

What's included

RootModelRegistry.get_registered_names()

• New override that unconditionally returns [""]
• The root's semantic contract — I am the empty-prefix anchor of every path — is unconditional. It does not depend on which Python process constructed this object or whether this instance happens to be the live singleton
• Placing the override directly on RootModelRegistry rather than patching the base-class identity check keeps the fix surgical: sub-registry resolution (_was_registered + orphan branch) is untouched

Why the existing _was_registered orphan branch does not cover this

• Sub-registries correctly serialize _was_registered=True and their chains survive pickling intact — the orphan branch short-circuits at the first orphaned node and never reaches the root
• RootModelRegistry is created via model_construct(), so _was_registered stays False and name is "" (falsy); even with _was_registered=True, the orphan branch guard self._was_registered and self.name would not fire for the root
• No change to _was_registered or the orphan branch is needed or made

What's NOT included

• No change to ModelRegistry.get_registered_names() — the base-class logic for live sub-registries and the include_orphaned path is correct and unmodified
• No change to pickling/unpickling behavior — the fix tolerates deserialization rather than preventing it
• No change to _REGISTRY_ROOT construction or the ModelRegistry.root() classmethod — the singleton pattern is preserved for all in-process uses

Tests

test_get_registered_names_survives_pickling (new, TestRegistry)

• Registers a model at /foo/bar/baz in a nested 3-level sub-registry chain
• Pickles and unpickles the model via pickle.dumps / pickle.loads, simulating Ray worker serialization of a standalone blob
• Asserts get_registered_names() still returns ['/foo/bar/baz'] on the deserialized instance

[github-actions]

Test Results

651 tests  +1   649 ✅ +1   1m 44s ⏱️ ±0s
  1 suites ±0     2 💤 ±0 
  1 files   ±0     0 ❌ ±0 

Results for commit 631cc87. ± Comparison against base commit e3a8b94.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.98%. Comparing base (e3a8b94) to head (631cc87).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #200      +/-   ##
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- Coverage   96.00%   95.98%   -0.02%     
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  Files         140      140              
  Lines        9827     9841      +14     
  Branches      569      569              
==========================================
+ Hits         9434     9446      +12     
- Misses        275      276       +1     
- Partials      118      119       +1     

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