fix(resolver): port v5.10.3 state isolation + collector + context fixes

[allmonday]

Summary

Three bug-fix ports from pydantic-resolve v5.10.3 that apply to nexusx's Resolver / Collector surface:

• #289-2 — concurrent resolve guard. Resolver holds per-call mutable state on self (_node_collectors, _loader_cache, the levels list inside _traverse). Two overlapping resolve() calls on the same instance clobbered each other and surfaced as a cryptic KeyError. Added a _in_resolve flag + try/finally that raises a clear RuntimeError instead.
• #293 — Collector subclass preservation. _phase_b_prepare_collectors hardcoded Collector(alias=alias, flat=flat) when instantiating per-node collectors, silently downgrading any ICollector implementation or Collector subclass with extra __init__ config (key_fn, n, dict-valued aggregator) to the base Collector. Now deepcopys the prototype, and the isinstance(param.default, Collector) check widened to ICollector so direct implementations are detected.
• #291 — context input validation. Resolver(context={}) was silently coerced to {} via context or {}, and non-dict values slipped through to confuse _traverse. Now rejects empty dict with ValueError and non-dict with TypeError at __init__.

The other two v5.10.3 fixes were evaluated and intentionally skipped — see Test plan notes.

Test plan

• tests/test_resolver_concurrency.py — 4 new tests for the concurrent-call guard
• tests/test_context.py::TestCollectorSubclass — 5 new tests migrated from pydantic-resolve test_collector_subclass.py
• tests/test_resolver_context_validation.py — 8 new tests for context validation (5 parametrized)
• Full suite: 1025 passed, 6 skipped (6 skipped are pre-existing Python 3.14-only PEP 649 tests)
• ruff check clean

N/A fixes (documented for reviewers):

• #290 (Profile.Timer.timeset leak) — nexusx has no Timer / profile module; only uses logging.
• #292 (inherited __pydantic_resolve_expose__ via getattr MRO false-positive) — nexusx's scan_expose_fields reads model_fields[field].metadata directly, not getattr(kls, '__xxx__'), so the MRO false-positive doesn't exist.
• #289-1 (object_level_collect_alias_map_store accumulating across calls) — already handled: _node_collectors.clear() and _loader_cache.clear() run at the top of every resolve().
• #289-3 (dict.get(k, expensive_default) evaluating default eagerly) — no such pattern in nexusx's resolver (audited every .get( call).

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