feat: multi-provider support — Ollama Cloud + OpenCode Go

- Account.Provider field (ollama-cloud | opencode-go), backward compatible
- Model-based routing: common models served by combined pool, unique models
  routed to their provider only
- /go/v1/* path forces OpenCode Go provider (prefix stripped upstream)
- Merged /v1/models endpoint returns union of both catalogs (44 models)
- Failover: 429/402 → cooldown + failover; 5xx → retry without cooldown
- CLI: accounts add --provider flag, list shows provider column
- Body buffering: request body buffered (8 MiB cap) for failover replay
- opencode integration: unified provider 'oc' with all merged models
- 64 tests pass (unit + integration)
- Verified: glm-5 → ollama, mimo-v2.5 → go, gpt-oss:20b → ollama

internal/proxy/balancer_test.go

@@ -11,11 +11,25 @@ import (
 func mkAccts(n int) []config.Account {
 	out := make([]config.Account, n)
 	for i := range out {
-		out[i] = config.Account{ID: "id" + string(rune('a'+i)), Name: "a" + string(rune('a'+i)), APIKey: "k" + string(rune('a'+i))}
+		out[i] = config.Account{
+			ID:       "id" + string(rune('a'+i)),
+			Name:     "a" + string(rune('a'+i)),
+			APIKey:   "k" + string(rune('a'+i)),
+			Provider: config.ProviderOllamaCloud,
+		}
 	}
 	return out
 }
 
+func mkMixedAccts() []config.Account {
+	return []config.Account{
+		{ID: "idO1", Name: "o1", APIKey: "k1", Provider: config.ProviderOllamaCloud},
+		{ID: "idO2", Name: "o2", APIKey: "k2", Provider: config.ProviderOllamaCloud},
+		{ID: "idG1", Name: "g1", APIKey: "k3", Provider: config.ProviderOpenCodeGo},
+		{ID: "idG2", Name: "g2", APIKey: "k4", Provider: config.ProviderOpenCodeGo},
+	}
+}
+
 func TestBalancer_Next_RoundRobin(t *testing.T) {
 	b := NewBalancer(mkAccts(3), 60*time.Second)
 	seen := make(map[string]int)
@@ -158,3 +172,67 @@ func TestBalancer_CooldownExpires(t *testing.T) {
 		t.Errorf("Next after cooldown expiry: %v", err)
 	}
 }
+
+func TestBalancer_NextFor_FiltersByProvider(t *testing.T) {
+	accts := mkMixedAccts()
+	b := NewBalancer(accts, 60*time.Second)
+	// Request only OpenCode Go accounts: every result must be a Go account.
+	seen := map[string]int{}
+	for i := 0; i < 8; i++ {
+		a, err := b.NextFor([]config.ProviderType{config.ProviderOpenCodeGo})
+		if err != nil {
+			t.Fatalf("NextFor %d: %v", i, err)
+		}
+		if a.Provider != config.ProviderOpenCodeGo {
+			t.Errorf("NextFor returned provider %q, want opencode-go", a.Provider)
+		}
+		seen[a.ID]++
+	}
+	if len(seen) != 2 {
+		t.Errorf("expected 2 distinct go accounts, got %d", len(seen))
+	}
+}
+
+func TestBalancer_NextFor_MixedPool(t *testing.T) {
+	accts := mkMixedAccts()
+	b := NewBalancer(accts, 60*time.Second)
+	// Eligible = both providers: all 4 accounts should be reachable.
+	seen := map[string]bool{}
+	for i := 0; i < 16; i++ {
+		a, err := b.NextFor([]config.ProviderType{
+			config.ProviderOllamaCloud, config.ProviderOpenCodeGo,
+		})
+		if err != nil {
+			t.Fatalf("NextFor %d: %v", i, err)
+		}
+		seen[a.ID] = true
+	}
+	if len(seen) != 4 {
+		t.Errorf("expected 4 distinct accounts in mixed pool, got %d", len(seen))
+	}
+}
+
+func TestBalancer_NextFor_NilEligibleReturnsAll(t *testing.T) {
+	accts := mkMixedAccts()
+	b := NewBalancer(accts, 60*time.Second)
+	seen := map[config.ProviderType]bool{}
+	for i := 0; i < 16; i++ {
+		a, err := b.NextFor(nil)
+		if err != nil {
+			t.Fatalf("NextFor nil %d: %v", i, err)
+		}
+		seen[a.Provider] = true
+	}
+	if !seen[config.ProviderOllamaCloud] || !seen[config.ProviderOpenCodeGo] {
+		t.Errorf("nil eligible should include both providers, got %v", seen)
+	}
+}
+
+func TestBalancer_NextFor_NoEligibleAccounts(t *testing.T) {
+	// Only Ollama accounts configured, but requesting Go.
+	b := NewBalancer(mkAccts(2), 60*time.Second)
+	_, err := b.NextFor([]config.ProviderType{config.ProviderOpenCodeGo})
+	if _, ok := err.(ErrAllCooldown); !ok && err == nil {
+		t.Errorf("NextFor with no eligible accounts: %v, want error or ErrAllCooldown", err)
+	}
+}