feat: add non-streaming LLM API, tool registry, and web_search tool

Replace SSE streaming with non-streaming JSON for Anthropic API.
Add llm_chat_tools() returning structured llm_response_t with text
and tool_use blocks. Implement tool registry with dispatch-by-name
and web_search tool via Brave Search API (direct + proxy support).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

main/llm/llm_proxy.h

@@ -1,7 +1,11 @@
 #pragma once
 
 #include "esp_err.h"
+#include "cJSON.h"
 #include <stddef.h>
+#include <stdbool.h>
+
+#include "mimi_config.h"
 
 /**
  * Initialize the LLM proxy. Reads API key and model from NVS.
@@ -29,3 +33,36 @@ esp_err_t llm_set_api_key(const char *api_key);
  * Save the model identifier to NVS.
  */
 esp_err_t llm_set_model(const char *model);
+
+/* ── Tool Use Support ──────────────────────────────────────────── */
+
+typedef struct {
+    char id[64];        /* "toolu_xxx" */
+    char name[32];      /* "web_search" */
+    char *input;        /* heap-allocated JSON string */
+    size_t input_len;
+} llm_tool_call_t;
+
+typedef struct {
+    char *text;                                  /* accumulated text blocks */
+    size_t text_len;
+    llm_tool_call_t calls[MIMI_MAX_TOOL_CALLS];
+    int call_count;
+    bool tool_use;                               /* stop_reason == "tool_use" */
+} llm_response_t;
+
+void llm_response_free(llm_response_t *resp);
+
+/**
+ * Send a chat completion request with tools to Anthropic Messages API (streaming).
+ *
+ * @param system_prompt  System prompt string
+ * @param messages       cJSON array of messages (caller owns)
+ * @param tools_json     Pre-built JSON string of tools array, or NULL for no tools
+ * @param resp           Output: structured response with text and tool calls
+ * @return ESP_OK on success
+ */
+esp_err_t llm_chat_tools(const char *system_prompt,
+                         cJSON *messages,
+                         const char *tools_json,
+                         llm_response_t *resp);