Has the Rashi text on Yshayahu 7:14 been corrupted? His note on the word הָרָה appears to say: לשון עתיד הוא, but, in this verse, הָרָה is an adjective ("pregnant") rather than a verb (part of the verb "to conceive"), and adjectives dont have a tense! I say that הָרָה is an adjective rather than a verb for the following reason: the associated noun (הָעַלְמָה) is feminine and, if הָרָה were a verb, it would be a masculine form—the corresponding feminine form would have to be הָרְתָה. On the other hand, הָרָה can also be a feminine adjective and this must therefore be the case in the verse under consideration.

Rashi himself answers your question, for he cites 2 verses from Judges 13 regarding the birth of Samson, where in v. 3 there it says in the future she will become pregnant, and then again in v. 5 again in the future, but this time: הרה, just like in Yeshayahu, she will become pregnant, and both are feminine. הרתה is in the past.