bluetooth: Classic: hfp_hf: make at_get_string() return const char *

Change at_get_string() return type to 'const char *'. Correspondingly,
change string parameters in HFP HF callbacks and related functions
from 'char *' to 'const char *' to reflect that these strings are
read-only data returned from AT command parsing.

- Update bt_hfp_hf_cb callback signatures for clip, operator,
  call_waiting, and textual_representation
- Update at_get_string() return type to 'const char *'
- Update local variables in hfp_hf.c handlers to use 'const char *'
- Update shell callback implementations to match new signatures
- Change NULL check style from '!text' to 'text == NULL' for consistency
- Remove unnecessary initialization of 'number' variable in clcc_handle

This improves const-correctness and prevents accidental modification
of AT command response strings.

Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>