Support structuredContent

[trufae]

The MCP protocol supports returning data in plaintext, which is the content field, and that's the default one that we support. But it also supports responding the tool calls with JSON that is embedded inside the structured content field. Not all the agents support this, but it will be good if R2 MCP was able to respond all the tool calls with the plaintext output in the content field, as well as the JSON result in the structured content.

[trufae]

In the current worktree, the text-only paths are the ones going through jsonrpc_tooltext_response(), tool_cmd_response(), or count_response() in src/tools.c and src/tools.c. Those bypass structuredContent.

Fully Not Handled
These normal success paths are still text-only:

open_file, close_file, list_functions, list_decompilers, show_info, show_function_details, get_current_address, list_entrypoints, calculate, set_comment, set_function_prototype, get_function_prototype, analyze, disassemble, use_decompiler, xrefs_to, rename_flag, rename_function, get_pid, dump_registers, hexdump, memory_map_here, alloc_memory, change_memory_protection, search, lookup_address, lookup_export, lookup_symbol, run_javascript, run_frida_script, list_sessions, open_session, close_session.

Notable references: open_file special-case at src/tools.c, close_file at src/tools.c, and many simple command tools use tool_cmd_response().

Partially Handled
These are mode-aware for normal list output, but count=true still returns text-only through count_response():

list_functions_tree, list_libraries, list_imports, list_exports, list_sections, list_memory_maps, list_symbols, list_methods, list_classes, list_strings, list_all_strings, list_files, list_threads, list_heap_allocations.

Looks Handled
These use the mode-aware paginated helper on the main success path:

run_command, run_script, decompile_function, disassemble_function.

So the largest missing piece is converting tool_cmd_response() and count_response() to mode-aware equivalents, then fixing the open_file dispatcher special-case.