[codex] Guard upb extension promotion by wire type

[kuranikaran]

Summary

Guard lazy message-extension promotion so a same-number unknown field is only promoted when its stored wire type is length-delimited. This keeps non-delimited unknowns on the existing skip path instead of capturing them and re-decoding them as embedded messages.

Root cause

upb_Message_GetOrPromoteExtension() matched unknown fields by field number only before calling upb_MiniTable_ParseUnknownMessage(). A same-number unknown with a varint or fixed wire type could be interpreted as a length-delimited message payload, causing the varint value bytes to be consumed as a message length.

Tests

• Added PromoteIgnoresWrongWireType covering ModelExtension1.model_ext encoded with a varint wire type and expecting kUpb_GetExtension_NotPresent.
• Verified with a standalone clang build using -fsanitize=address,undefined: wrong-wire input returns kUpb_GetExtension_NotPresent; length-delimited empty-message input returns kUpb_GetExtension_Ok with non-null msg_val.

Follow-up

The legacy PromoteUnknownToMessage, PromoteUnknownToMessageArray, and PromoteUnknownToMap paths can reach the same parse helper via FindUnknown; they can receive the same wire-type guard in a follow-up.