fix(util): harden utility edge cases

[chaoming0625]

Summary

• repair stale util package export by binding breakpoint_if lazily
• harden BoundedCache, DictManager, DotDict, flatten/nest helpers, filters, and struct helpers against audited edge cases
• add regression coverage for wildcard exports, cache bounds/messages, JAX key paths, stale identity indexes, direct DotDict mutation, path conflicts, dataclass metadata, and mixed-key FrozenDict behavior

Tests

• pytest brainstate/util -q
• pytest brainstate/_state_test.py brainstate/transform/_debug_test.py::TestBreakpointIf brainstate/transform/_find_state_test.py brainstate/transform/_mapping_core_test.py -q
• pytest brainstate -q

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[codecov]

Codecov Report

❌ Patch coverage is 82.97872% with 8 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
brainstate/util/_others.py	71.42%	2 Missing and 2 partials ⚠️
brainstate/util/__init__.py	33.33%	2 Missing ⚠️
brainstate/util/_cache.py	66.66%	1 Missing and 1 partial ⚠️

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