chore(review): small hardening items in MCP guard, RPC client, sandbox runner, CSV export, file launch #1565

Small findings from a review of areas with no open issues. Split into separate issues if any grows.

• MCP SQL guard (lib/core/mcp/mcp_sql_guard.dart:40-49): _forbiddenFunctions lacks QUERY_TO_XML* / CURSOR_TO_XML / TABLE_TO_XML*, PG_ADVISORY_*, PG_SLEEP, LO_*, NEXTVAL / SETVAL. No bypass found (the read-only session is the second layer), this is defence in depth. _explainForbidden includes REPLACE, which refuses EXPLAIN SELECT REPLACE(...) (false positive).
• McpSocketHost (lib/core/mcp/mcp_socket_host.dart:90-95): a line before authentication has no length limit and the number of unauthenticated sockets is unbounded (loopback only, 5 s handshake).
• JsonRpcStdioClient.sendRequest (lib/core/extensions/rpc/json_rpc_stdio_client.dart:86-95): _stdin.writeln is outside the try; on a closed sink it throws synchronously and leaves _pending[id] behind.
• SandboxProcessRunner.start (lib/core/extensions/sandbox/sandbox_process_runner.dart:123-145): the scratch directory is created before SandboxLaunchCommand.build; an exception there leaks it.
• CSV export (lib/core/csv/result_grid_csv.dart, save_result_grid_csv.dart): no UTF-8 BOM (Excel misreads Cyrillic), no formula-injection guard for cells starting with = + - @, \n instead of CRLF while documented as RFC 4180, the write error cause is swallowed.
• FileLaunchService (lib/core/platform/file_launch_service.dart): hasPendingTarget duplicates hasPendingTargets; path comparison in resolveOrRegisterSqliteConnection is case-sensitive (duplicate connections on Windows/macOS).