[Open Codesign] Align connection test and actual invocation paths for provider parity

# [Feature]: 对齐“测试连接”和“实际调用”链路 / Ensure connection-test and actual-invocation parity ## Problem / pain point / 问题 当前“测试连接”与“实际调用”并不总是走同一套有效 provider contract，因此容易出现误判： - 测试连接通过，但真实 `generate` 失败 - 测试连接失败，但真实调用其实可用 - `/models` 探测、wire 选择、baseUrl 规范化、keyless 判定、OAuth / Bearer 认证处理不完全一致 - active provider 的解析、renderer 传入值覆盖、真实调用时的 baseUrl snap-to-config 逻辑，并没有完整复用到测试链路 Today the connection-test path and the real invocation path do not always share the same effective provider contract, which creates false positives and false negatives: - connection test passes while real `generate` fails - connection test fails even though real invocation works - `/models` probing, wire selection, baseUrl normalization, keyless handling, and OAuth / Bearer auth are not fully aligned - active-provider resolution and runtime-only config snapping are not consistently reused by connection tests ## Proposed solution / 方案 把“测试连接”升级成“对真实调用链路的低成本验证”，而不是一条独立的近似探测路径。 Turn connection testing into a low-cost va