fail fast when `uponReceiving` is not set

[sku0x20]

Checklist

Before making a feature request, I have:

• [ x] Searched the issues to check that this feature hasn't been requested before
• [ x] Checked the documentation to see if it is possible to do what I want already

Feature description

when adding interactions in PactV4, fail fast if uponReceiving is not set.
from what i found its a required parameter.
if it can't be done; pls add it in the docs.

Use case

save debug time by failing fast if uponReceiving is not set.

Issue

my pact file was always having the last interaction no matter how i run the tests.
i wasted 2 days of debugging, thinking it was different pact instances causing the issue, trying to have global setup, finding PactfileWriteMode can help but not there in v4, removed cause rust core does merging by default, looking at debug traces, which says merging, yet it's not happening. after all this finding uponReceiving by chance.
very frustrated rn. :(

imho, instead of uponReceiving just putting description in builder would have helped too.

anyways, as a stupid person i would really appreciate some basic stupidity checks. :(

[mefellows]

Yep, that's fair. The types (that generaly try to enforce this) can't be changed now as that would be a breaking change, but we can check inside the function calls.