r2plugin: prevent command injection in signature fallback apply path

[trufae]

Motivation

• The fallback write-back path executed r_core_cmdf_at(..., "afs %s", signature) with an inferred signature string that could include an unsanitized function name coming from untrusted binaries, enabling radare2 command injection via separators like ;, |, &, newlines, or !.

Description

• Added signature_contains_cmd_separator() in r2plugin/r_anal_sleigh.c to detect dangerous separator characters in the signature string before executing the fallback command.
• Updated apply_inferred_signature() to skip the r_core_cmdf_at fallback when a separator is detected, mark the apply result as failed, and write a descriptive reason instead of executing the unsafe command.
• This change only hardens the fallback (command) path and preserves the existing API-first r_anal_str_to_fcn apply-and-verify flow and behavior when no unsafe characters are present.
• Blocked separators: ;, |, &, \n, \r, and !.

Testing

• Ran the unit test test_format_afs_signature via cargo test -p r2sleigh-plugin --features x86 test_format_afs_signature -- --nocapture, which compiled the workspace and passed (1 test passed).

---

Codex Task

[trufae]

actually the right fix is just to prefix with a single quote the afs command. no need to filter all that. so pls take that into account and do the right fix by hand