feat(experiment): add opt-in model routing for evaluators

[pdebjyot]

Closes #323
Related: #88

Description

Adds ModelRouter and RoutingRule (new module strands_evals/model_router.py), plus an optional model_router= parameter on Experiment, so evaluation cost/accuracy can be matched to evaluator complexity: fast models for structural/rubric checks, strong models for nuanced judgment — including case-dependent escalation (e.g., long traces get a stronger judge).

Semantics:

• Opt-in: no model_router → behavior unchanged.
• Explicit wins: an evaluator constructed with model=... is never re-routed.
• First matching rule wins; rules match on evaluator type (class or name) plus an optional per-case condition predicate. No match falls through to an optional default_model.
• Routing returns a shallow copy of the evaluator carrying the routed model, so shared instances are never mutated across concurrent workers. Evaluators without a model attribute (deterministic) pass through untouched.
• A raising condition logs a warning and skips the rule instead of failing the evaluation.
• model_router is intentionally excluded from Experiment.to_dict()/from_dict() since routing conditions are arbitrary callables.

Example

from strands_evals import Experiment, ModelRouter, RoutingRule

router = ModelRouter(rules=[
    RoutingRule(
        evaluator_types=[ToolSelectionAccuracyEvaluator, ToolParameterAccuracyEvaluator],
        model="us.anthropic.claude-haiku-4-5-20251001-v1:0",
    ),
    RoutingRule(
        evaluator_types=[GoalSuccessRateEvaluator],
        model="us.anthropic.claude-opus-4-1-20250805-v1:0",
        condition=lambda case: len(str(case.actual_trajectory or "")) > 10_000,
    ),
])

experiment = Experiment(cases=cases, evaluators=evaluators, model_router=router)

Testing

• 12 new tests: rule matching by class/name, condition gating, condition-exception safety, first-match-wins, default-model fallback, explicit-model preservation, no-mutation of shared instances, deterministic-evaluator passthrough, and two end-to-end Experiment integration tests asserting the judge agent is constructed with the routed model (and unchanged without a router).
• Full evaluator + experiment unit suites pass (454 tests); ruff check and ruff format clean.

Checklist

• Non-breaking change (opt-in)
• Unit + integration tests added
• Conventional commit
• Lint/format clean