[finsh][shell] Print FinSH prompt with a fixed format string

[Copilot]

The FinSH/MSH prompt path treated the prompt buffer as a rt_kprintf() format string. When the working directory contained format specifiers, the prompt could leak stack data or crash instead of printing the path literally.

• Problem

  • finsh_get_prompt() builds the prompt from static text plus the current working directory.
  • components/finsh/shell.c then printed that buffer directly with rt_kprintf(FINSH_PROMPT), making directory names part of the format string.

• Change

  • Update both direct prompt-printing call sites in components/finsh/shell.c to use a fixed format string.
  • Keep prompt construction and shell behavior unchanged; only the sink is hardened.

• Effect

  • Working directory names are printed verbatim in the prompt.
  • Format specifiers in directory names are no longer interpreted by rt_kprintf().

/* before */
rt_kprintf(FINSH_PROMPT);

/* after */
rt_kprintf("%s", FINSH_PROMPT);