Port Python agentic search to Go (nav service, harness, tools) (#17702)

Port Python rag/advanced_rag agentic search to Go: ES-backed dataset-nav
service, agentic-search harness, and agent tools.

Includes agentic-search port plan and self-review docs.

internal/agent/chat/chat.go

@@ -0,0 +1,110 @@
+//
+//  Copyright 2026 The InfiniFlow Authors. All Rights Reserved.
+//
+//  Licensed under the Apache License, Version 2.0 (the "License");
+//  you may not use this file except in compliance with the License.
+//  You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+//  Unless required by applicable law or agreed to in writing, software
+//  distributed under the License is distributed on an "AS IS" BASIS,
+//  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+//  See the License for the specific language governing permissions and
+//  limitations under the License.
+//
+
+// Package chat is a dependency-light home for the LLM chat-invoker interface
+// and its package-level singleton. It lives here (leaf, importing only eino
+// schema + gorm) so that both internal/agent/component (which owns the
+// production eino-based invoker) and internal/agent/tool / internal/agent/harness
+// (which need to call the LLM for routing/selection) can depend on it without
+// forming an import cycle. The production invoker is registered at boot via
+// SetDefaultInvoker.
+package chat
+
+import (
+	"context"
+	"sync"
+
+	"github.com/cloudwego/eino/schema"
+	"gorm.io/gorm"
+)
+
+// Invoker abstracts a chat-model call so callers can inject a stub in tests and
+// production flows through the eino bridge. This is the shared seam for the LLM
+// component and the agentic-search harness/tools.
+type Invoker interface {
+	Invoke(ctx context.Context, db *gorm.DB, req Request) (*Response, error)
+}
+
+// Request is the minimal surface needed to dispatch a chat call.
+type Request struct {
+	Driver           string
+	ModelName        string
+	APIKey           string
+	BaseURL          string
+	Messages         []schema.Message
+	Temperature      *float64
+	TopP             *float64
+	PresencePenalty  *float64
+	FrequencyPenalty *float64
+	MaxTokens        *int
+	Thinking         string // "enabled" | "disabled" | ""
+}
+
+// Response is the result of a chat call.
+type Response struct {
+	Content  string
+	Thinking string
+	Model    string
+	Stopped  bool
+	Tokens   int
+}
+
+// ErrNotConfigured is returned by GetDefaultInvoker when no production invoker
+// has been installed. Callers should treat it as "no chat model available".
+var ErrNotConfigured = &configError{"chat: default invoker not configured"}
+
+type configError struct{ msg string }
+
+func (e *configError) Error() string { return e.msg }
+
+var (
+	mu           sync.RWMutex
+	inst         Invoker
+	defaultModel string
+)
+
+// SetDefaultInvoker installs the (production or test) invoker. Pass nil to
+// restore the "not configured" state.
+func SetDefaultInvoker(inv Invoker) {
+	mu.Lock()
+	defer mu.Unlock()
+	inst = inv
+}
+
+// SetDefaultModelName records the tenant-default chat model name. The production
+// invoker uses it when a Request omits ModelName, so harness/agentic-search LLM
+// calls work without threading model config through every node.
+func SetDefaultModelName(name string) {
+	mu.Lock()
+	defer mu.Unlock()
+	defaultModel = name
+}
+
+// GetDefaultModelName returns the configured default model name ("" if unset).
+func GetDefaultModelName() string {
+	mu.RLock()
+	defer mu.RUnlock()
+	return defaultModel
+}
+
+// GetDefaultInvoker returns the installed invoker. It returns nil when no
+// invoker has been installed (e.g. before server bootstrap or in tests that do
+// not need the LLM).
+func GetDefaultInvoker() Invoker {
+	mu.RLock()
+	defer mu.RUnlock()
+	return inst
+}