[grouping] stack.module in LHS of fingerprinting rule sometimes different from stack.module of RHS

[lobsterkatie]

A customer reports that with a fingerprinting rule like this

stack.module="someModule" -> {{ default }} title="{{ stack.module }}"

they end up with issues titled "SomeOtherModule" rather than "SomeModule".

It's possible that the module value is getting changed between the time we apply fingerprint rules and the time we compute the title. It's also possible that we're looking at two different stacks. In the customer's case, they have chained errors (in Java, though I'm not sure that matters, since the payload schema should be the same across SDKs), and it seems like we can't decide which of those errors to use, and use one for matching and another for titling.

Regardless, this is obviously unexpected behavior as far as fingerprinting rules goes, and we should fix it.

Note: This might be (?) semi-related to #59679, though that was triggered by a change in the data sent by the SDK, and is about grouping, not titling. (In the above customer's case, grouping actually seems to be working just fine, in that they're getting a bunch of "SomeOtherModule" issues, because those events are grouping on whatever module should be in the title rather than "SomeOtherModule". For them, if the title could be fixed, all would be working as it should.) They do share chained errors as part of the problem, though, so worth at least keeping in the back of our minds.