feat: same-provider retry for transient upstream failures

Adds spec §1's opt-in retry engine: RetryConfig (top-level, zero value
disabled) and retryPolicy (retry.go), wired into upstreamJSON — the
single HTTP chokepoint every adapter's chat/embeddings/listModels call
already funneled through. A transient failure (net error, 429, 5xx)
retries up to Attempts times with exponential backoff capped at 2s,
honoring a qualifying Retry-After header on 429, draining and closing
each failed attempt's body before retrying. Never retries a non-429 4xx,
a translate error, or after context cancellation.

Because upstreamJSON returns before any adapter forwards a byte to the
client (forwardStream/forwardJSON run only after it returns), the
'zero bytes reached client' safety rule and the 'streaming retries only
the initial exchange' rule both hold by construction — no adapter
streaming logic needed to change.

buildAdapters(cfg) keeps its v0.1 signature: it derives the retryPolicy
from cfg.Retry internally rather than taking it as a parameter, so
every existing direct call (15+ across provider_openai_test.go) keeps
compiling unmodified. Every v0.1 test passes byte-identically, without
edits.

provider_anthropic.go

@@ -19,7 +19,13 @@ const (
 
 // anthropicAdapter is the providerAdapter for provider type "anthropic".
 type anthropicAdapter struct {
-	client      *http.Client
+	client *http.Client
+	// retry is nil for every adapter built by a bare newAnthropicAdapter
+	// call (every v0.1 test, and any caller that never wires one) —
+	// nil-safe, per retryPolicy.do's own contract, so leaving it unset is
+	// exactly v0.1's one-attempt behavior. buildAdapters (providers.go)
+	// sets it from cfg.Retry for production use.
+	retry       *retryPolicy
 	adapterName string
 	baseURL     string
 	apiKey      string
@@ -106,7 +112,7 @@ func (a *anthropicAdapter) chatCompletion(ctx context.Context, w http.ResponseWr
 		hdr.Set("Accept", "text/event-stream")
 	}
 
-	resp, err := upstreamJSON(ctx, a.client, http.MethodPost, a.baseURL+anthropicMessagesPath, hdr, body)
+	resp, err := upstreamJSON(ctx, a.client, http.MethodPost, a.baseURL+anthropicMessagesPath, hdr, body, a.retry)
 	if err != nil {
 		return usage{}, err
 	}
@@ -200,7 +206,7 @@ func (a *anthropicAdapter) embeddings(_ context.Context, _ http.ResponseWriter,
 // falls back to a provider's explicitly configured models on error rather
 // than this adapter guessing at a fallback itself.
 func (a *anthropicAdapter) listModels(ctx context.Context) ([]string, error) {
-	resp, err := upstreamJSON(ctx, a.client, http.MethodGet, a.baseURL+anthropicModelsPath, a.requestHeaders(false), nil)
+	resp, err := upstreamJSON(ctx, a.client, http.MethodGet, a.baseURL+anthropicModelsPath, a.requestHeaders(false), nil, a.retry)
 	if err != nil {
 		return nil, err
 	}