feat: Add ExtraBody/ExtraFields support for custom OpenAI-compatible API parameters

This change enables passing custom parameters to OpenAI-compatible API
providers via the provider config's extra_body field. The implementation
maps extra_body to Fantasy's extra_fields, which are then passed through
to the underlying OpenAI SDK's SetExtraFields() method.

Key changes:
- Added extra_body mapping to extra_fields in getProviderOptions()
- Merge extra_body options into provider options chain
- Uses charm.land/fantasy v0.1.1 with ExtraFields support

This enables advanced provider features like Z.AI GLM's thinking mode:
{
  "providers": {
    "z.ai": {
      "extra_body": {
        "thinking": {
          "type": "enabled",
          "budget_tokens": 26214
        }
      }
    }
  }
}

The extra_body parameters are correctly passed through the HTTP request
to the provider API.

Author: Jesssullivan

go.mod

@@ -50,6 +50,11 @@ require (
 	mvdan.cc/sh/v3 v3.12.1-0.20250902163504-3cf4fd5717a5
 )
 
+require (
+	github.com/anthropics/anthropic-sdk-go v1.14.0 // indirect
+	google.golang.org/genai v1.32.0 // indirect
+)
+
 require (
 	cloud.google.com/go v0.116.0 // indirect
 	cloud.google.com/go/auth v0.17.0 // indirect
@@ -58,7 +63,6 @@ require (
 	github.com/Azure/azure-sdk-for-go/sdk/azcore v1.17.0 // indirect
 	github.com/Azure/azure-sdk-for-go/sdk/internal v1.10.0 // indirect
 	github.com/andybalholm/cascadia v1.3.3 // indirect
-	github.com/anthropics/anthropic-sdk-go v1.14.0 // indirect
 	github.com/aws/aws-sdk-go-v2 v1.39.3 // indirect
 	github.com/aws/aws-sdk-go-v2/aws/protocol/eventstream v1.6.3 // indirect
 	github.com/aws/aws-sdk-go-v2/config v1.27.27 // indirect
@@ -158,7 +162,6 @@ require (
 	golang.org/x/text v0.30.0
 	golang.org/x/time v0.12.0 // indirect
 	google.golang.org/api v0.239.0 // indirect
-	google.golang.org/genai v1.32.0 // indirect
 	google.golang.org/genproto/googleapis/rpc v0.0.0-20250603155806-513f23925822 // indirect
 	google.golang.org/grpc v1.74.2 // indirect
 	google.golang.org/protobuf v1.36.10 // indirect

internal/agent/coordinator.go

@@ -142,6 +142,7 @@ func getProviderOptions(model Model, providerCfg config.ProviderConfig) fantasy.
 	cfgOpts := []byte("{}")
 	providerCfgOpts := []byte("{}")
 	catwalkOpts := []byte("{}")
+	extraBodyOpts := []byte("{}")
 
 	if model.ModelCfg.ProviderOptions != nil {
 		data, err := json.Marshal(model.ModelCfg.ProviderOptions)
@@ -164,9 +165,24 @@ func getProviderOptions(model Model, providerCfg config.ProviderConfig) fantasy.
 		}
 	}
 
+	// Merge extra_body from provider config (for Z.AI GLM thinking mode, etc.)
+	// Map it to extra_fields which Fantasy's ExtraFields expects
+	if providerCfg.ExtraBody != nil && len(providerCfg.ExtraBody) > 0 {
+		extraBodyData := map[string]any{
+			"extra_fields": providerCfg.ExtraBody,
+		}
+		data, err := json.Marshal(extraBodyData)
+		if err == nil {
+			extraBodyOpts = data
+		} else {
+			slog.Warn("Failed to marshal extra_body from provider config", "error", err)
+		}
+	}
+
 	readers := []io.Reader{
 		bytes.NewReader(catwalkOpts),
 		bytes.NewReader(providerCfgOpts),
+		bytes.NewReader(extraBodyOpts),
 		bytes.NewReader(cfgOpts),
 	}