feat(providers): success-rate accounting

Track per-provider and per-(provider,upstream-model) upstream-attempt
outcomes using the existing fixed-window counter machinery (limits.go),
following mcp_a2a.go's per-target counter pattern: kind-embedded keys
(kindProvider="prov", kindProviderModel="provmodel") keep a provider
scope collision-safe against a user/group/MCP-target/agent of the same
name.

limiter.recordProviderAttempt(provider, model, resp, err) increments an
"attempts" counter on every call, and a "failures" counter only for a
provider-fault outcome — reusing retry.go's own isTransient(resp, err)
classification exactly (transport/connection error, HTTP 429, or a 5xx),
not forking it. Both counters are written at minute and day granularity
only; provider health is a now-and-today question, not a billing one, so
there is no month window. limiter.providerUsage batches every scope's
read into one storeGetMulti round trip, mirroring targetUsage's own
single-batch discipline.

Wired at every chokepoint an upstream HTTP attempt resolves:
- providers.go's upstreamJSON/upstreamRawBytes path (chat, embeddings,
  images, audio) via retry.go's retryPolicy.do, which now looks up an
  optional attemptRecorder from ctx and invokes it once per retry
  attempt (attempt-accounting: a request retried twice before
  succeeding reports three attempts, not one).
- routes_unified.go's runUnified (chat/embeddings, streaming and
  non-streaming alike — both funnel through the same upstreamJSON call).
- routes_media.go's three media handlers (images.generations,
  audio.speech, audio.transcriptions).
- routes_passthrough.go's proxyUpstream (native provider passthrough) —
  a single un-retried attempt, provider-level only: the upstream model
  lives in the response body, read only after the attempt resolves.

The attemptRecorder is threaded via context (providers.go's
withAttemptRecorder/attemptRecorderFromContext), not a new function
parameter on every call site — proxyUpstream and retryPolicy.do are
both shared with paths that must NOT record provider accounting
(registry.go's discovery listModels calls, and the MCP/A2A target
proxy, mcp_a2a.go's handleTargetProxy), so scoping via context keeps
their signatures unchanged and those callers correctly opt out by
simply never wrapping their context.

mcp_a2a_test.go

@@ -684,6 +684,16 @@ func TestHandleTargetProxy_JSONResponse_OnlyRequestCounterMoves(t *testing.T) {
 	if !ok || totalReq != 1 {
 		t.Errorf("total request/min counter = %d (ok=%v), want 1", totalReq, ok)
 	}
+
+	// Feature A (v0.22): handleTargetProxy/proxyUpstream never wrap the
+	// request context with an attemptRecorder — an MCP/A2A target proxy
+	// attempt against the "openai" MCP server's own upstream must never be
+	// mistaken for provider traffic, even though a provider of the same
+	// name happens to be configured too.
+	provAttempts, _ := gw.limiter.getCounter(kindProvider, "openai", metricProvAttempt, windowDay, time.Now())
+	if provAttempts != 0 {
+		t.Errorf("provider attempts/day = %d, want 0 — the MCP target proxy must never record provider accounting", provAttempts)
+	}
 }
 
 // --- config validation at construction ---