feat(adapters): auto-retry with exponential backoff [closes #118]

[EmersonBraun]

Summary

Phase 1 #118. Every adapter now retries transient failures by default — no caller code needed.

Retry policy

What	Behavior
HTTP 408, 429, 500, 502, 503, 504	retried with exponential backoff
Network errors (fetch throws)	retried
HTTP 4xx (except 408/429)	terminal — bad request / auth, won't get better
AbortError	terminal — caller cancelled
Retry-After header	respected (seconds or HTTP date)
Mid-stream errors	not retried — partial output already on the wire

Defaults: 3 attempts, base 500ms × 2^n with full jitter, max 8s per delay.

Configuration

import { openai } from '@agentskit/adapters'

const adapter = openai({
  apiKey: KEY,
  model: 'gpt-4o',
  retry: {
    maxAttempts: 5,
    baseDelayMs: 1000,
    maxDelayMs: 30_000,
    jitter: true,
    onRetry: ({ attempt, delayMs, reason }) =>
      console.log(`retry #${attempt} in ${delayMs}ms — ${reason}`),
  },
})

Opt out entirely: retry: { maxAttempts: 1 }.
Custom decision logic: retry: { retryOn: ({ response, error }) => ... }.

Standalone primitive

import { fetchWithRetry } from '@agentskit/adapters'

const res = await fetchWithRetry(
  signal => fetch('https://api.example.com', { signal }),
  controller.signal,
  { maxAttempts: 4 },
)

Wired into

• openai, anthropic, gemini, ollama, vercelAI
• All accept the same retry?: RetryOptions field on their config

Tests

13 new cases on @agentskit/adapters (was 50, now 63 passing):

• Success path (no retry)
• Terminal 4xx (401, etc.)
• 429 retry chain
• 5xx retry chain
• Retry-After (seconds + HTTP date)
• Exponential backoff math
• Exhaustion returns last response
• Abort propagation (rethrows AbortError)
• Network error retry chain
• Mid-attempt abort
• onRetry hook firing
• Custom retryOn predicate

Test plan

• Build succeeds
• All 63 tests pass
• Per-adapter config field threaded everywhere
• Standalone fetchWithRetry exported
• Reviewer: try a real OpenAI call with mock server returning 503 → success after retry

Refs #118 #211