feat(routing): add ensemble response synthesis

Feature request

Add an ensemble routing method that combines independent responses from multiple models into one synthesized answer.

Instead of selecting a single model for each request, Switchyard should send the request to multiple candidate models concurrently, then pass their drafts to a configured synthesizer model to produce the final response.

Motivation

Explore whether combining two smaller or less capable models can provide useful answer quality compared with a stronger single-model baseline, particularly for coding tasks. Quality, latency, and total upstream usage should be evaluated rather than assuming that an ensemble is better or cheaper.

Proposed behavior

• Configure candidate model targets and a synthesizer target per route.
• Generate candidate drafts concurrently.
• Give candidates independent output budgets.
• Require a configurable minimum number of successful candidates before synthesis.
• Synthesize usable drafts into a single response while preserving the caller-facing API and final-response streaming settings.
• Handle candidate failures explicitly and keep internal reasoning blocks out of synthesis inputs.

Implementation PR

PR #657: feat(routing): add ensemble response synthesis implements this method with two to four candidate targets, configurable synthesis, validation, and tests.

The PR also includes exploratory comparisons of SOL + Opus → SOL against Astra. These small-sample results are preliminary and do not establish general quality equivalence, superiority, or a cost advantage. Candidate usage must be included when comparing total cost.

Acceptance criteria

• An ensemble route invokes its candidates concurrently and synthesizes their usable drafts.
• Candidate budgets, synthesizer selection, and the minimum-success threshold are configurable and validated.
• Failure handling and final-response streaming behavior are covered by tests.
• Documentation explains configuration, additional upstream calls, and usage-accounting limitations.

This request is specifically for the ensemble routing method; frontend configuration is separate.

Start with PR #657, which implements the ensemble routing method, then review its validation and tests against this issue's acceptance criteria. Confirm candidate concurrency, configurable budgets and thresholds, failure handling, streaming behavior, documentation, and usage-accounting limitations before considering the work complete.