Fix duplicate empty-string alternative in continuation_token regex

[aaguiarz]

Description

In openfga/v1/openfga_service.proto around line 1838, the continuation_token field in ListStoresResponse has a duplicate ^$| in its validation pattern:

(validate.rules).string.pattern = "^$|^$|^[A-Za-z0-9-_]+={0,2}$"

All other continuation_token fields in the file use the correct pattern with a single empty-string alternative:

(validate.rules).string.pattern = "^$|^[A-Za-z0-9-_]+={0,2}$"

Fix

Replace:

"^$|^$|^[A-Za-z0-9-_]+={0,2}$"

with:

"^$|^[A-Za-z0-9-_]+={0,2}$"

Impact

No functional impact — the duplicate ^$| is redundant and the regex matches the same strings either way. This is a cleanup to keep the patterns consistent across the file.

[Siddhant-K-code]

@aaguiarz - we should fix this separately. As #240 is a large PR which might take time to land.