fix(modem): Harden AT library against command injection

[david-cermak]

AT-001: Undefined Behavior on Empty Token in generic_get_string [FIXED]
Severity: Medium | esp_modem_command_library.cpp:92

A malicious modem sending \n\n as a response creates an empty string_view token. The original code computed token.end() - 1 on an empty view — undefined behavior. A conforming compiler may optimize away the loop guard under UB assumptions.

Fix applied: Replaced the reverse-iterator loop with while (!token.empty() && (token.back() == '\r' || token.back() == '\n')).

AT-003: Wrong substr Length in Quote Stripping [FIXED]
Severity: Medium | esp_modem_command_library.cpp:266

A malicious modem sending +COPS: 1,0,X"Attacker",7 causes get_operator_name to extract extra characters from the response. operator_name.substr(quote1 + 1, quote2 - 1) uses quote2 - 1 as count, which is only correct when quote1 == 0. When the modem places characters before the first quote, the extracted name includes trailing data.

Fix applied: Changed to substr(quote1 + 1, quote2 - quote1 - 1) with a quote2 > quote1 guard.

AT-004: Missing Comma in get_network_system_mode Returns Wrong Value [FIXED]
Severity: Medium | esp_modem_command_library.cpp:573-578

A malicious modem sending +CNSMOD: 7 (no comma) causes int mode_pos = out.find(",") + 1 to yield mode_pos = 0 (since npos + 1 = 0). from_chars then parses from position 0, and if the modem crafts the line to start with a digit (e.g., by including it in a previous URC), an attacker-controlled value is returned.

Fix applied: Added explicit comma-not-found check; return FAIL if no comma.

AT-005: from_chars Out-of-Range Not Checked [FIXED]
Severity: Medium | Multiple locations

A malicious modem sending very large numeric values (e.g., +CSQ: 99999999999999999,3) triggers std::errc::result_out_of_range, but only errc::invalid_argument was checked. The output parameter is left unmodified per the C++17 spec — returning whatever stale/uninitialized value was in the caller's variable as if parsing succeeded.

Fix applied: Changed all from_chars error checks from == std::errc::invalid_argument to != std::errc{} to catch all error codes.

AT-006: Inverted from_chars Range on Truncated Battery Response [FIXED]
Severity: Medium | esp_modem_command_library.cpp:230

A malicious modem sending +CBC: 3. (truncated after the dot) causes from_chars(out.data() + dot_pos + 1, out.data() + out.size() - 1, ...) to receive an inverted range (begin > end) — undefined behavior.

Fix applied: Added range validation (dot_pos + 2 > out.size() → return FAIL) before the from_chars call.