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/tools/tool_registry.h

@@ -0,0 +1,34 @@
+#pragma once
+
+#include "esp_err.h"
+#include <stddef.h>
+
+typedef struct {
+    const char *name;
+    const char *description;
+    const char *input_schema_json;  /* JSON Schema string for input */
+    esp_err_t (*execute)(const char *input_json, char *output, size_t output_size);
+} mimi_tool_t;
+
+/**
+ * Initialize tool registry and register all built-in tools.
+ */
+esp_err_t tool_registry_init(void);
+
+/**
+ * Get the pre-built tools JSON array string for the API request.
+ * Returns NULL if no tools are registered.
+ */
+const char *tool_registry_get_tools_json(void);
+
+/**
+ * Execute a tool by name.
+ *
+ * @param name         Tool name (e.g. "web_search")
+ * @param input_json   JSON string of tool input
+ * @param output       Output buffer for tool result text
+ * @param output_size  Size of output buffer
+ * @return ESP_OK on success, ESP_ERR_NOT_FOUND if tool unknown
+ */
+esp_err_t tool_registry_execute(const char *name, const char *input_json,
+                                char *output, size_t output_size);