Fix fragile arguments.length check in findByEmailOrUsername

[SinhaS12]

Issue:

Fixes #3911
findByEmailOrUsername() used arguments.length === 2 to decide whether to run a case-insensitive query. This is fragile and can behave inconsistently when the function is called with extra arguments (e.g. wrappers) or different call styles.

Demo:

N/A (server-side logic change; no UI change).

Changes:

• Remove the arguments.length === 2 condition from the case-insensitive branch.
• Control case-insensitive behavior by explicitly checking options?.caseInsensitive.

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• has no typecheck errors (npm run typecheck)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123

[raclim]

Thanks @SinhaS12 and @clairep94!