Replace OpenAI and Anthropic SDK retries with local request retries that
preserve configured retry counts, enforce maxRetryDelayMs, and make retry
backoff interruptible through AbortSignal.
Alternatives considered: Racing SDK requests against AbortSignal would make
cancellation observable but would not let Pi enforce maxRetryDelayMs before
the SDK enters its retry sleep. Patching private SDK internals would be brittle,
while disabling provider retries entirely would ignore explicitly configured
maxRetries.
Also, align the custom OpenAI Codex retry loop with the documented
maxRetryDelayMs behavior. Oversized server-requested delays now fail immediately
instead of being clamped, allowing the visible, abortable agent-level retry
policy to handle the failure.
Fixes#6911
Move provider auth and OAuth flows onto pi-ai Models, compose models.json and extension overlays through ModelRuntime, and retain ModelRegistry as an extension compatibility facade.
