Support UTF-8 SMS submission using UCS-2

[dormancygrace]

Summary

• strip a leading + before semi-octet phone-number encoding and validate all destination digits
• automatically encode non-ASCII UTF-8 messages as UCS-2 with the single-message 70-character limit
• reject malformed UTF-8, unsupported non-BMP characters, invalid numbers, and oversized messages
• propagate PDU and modem submission errors instead of continuing with invalid state
• add exact GSM-7 and UCS-2 SMS-SUBMIT test vectors

This addresses the missing send-side support discussed in #12 and #14. ASCII messages keep the existing GSM-7 path.

Validation

• make -C pdu_lib test
• PDU encoder tests under ASan and UBSan
• cross-compiled for OpenWrt MIPS32r2/musl
• installed and exercised on a physical Fibocom NL678 modem; an international-number Cyrillic message was accepted by the modem as +CMGS: 8

The hardware test used the same Cyrillic text as the committed UCS-2 test vector.

[dormancygrace]

Hardware delivery follow-up: both UTF-8/UCS-2 test messages were confirmed received, not merely accepted by the modem.

Tested on a physical Fibocom NL678 with two international destinations (+7... and +972...). The modem returned +CMGS: 8 and +CMGS: 9; both recipients received the exact Cyrillic text.

[obsy]

I will test it with our (Polish) diacritics and let you know how it works.