refactor: fix line length and use cli for latent prior checks

[google-labs-jules]

This PR addresses a linting issue and standardizes error handling for latent model prior checks.

Changes:

• Refactoring: Split a long comment in tests/testthat/test-prior-latent-model.R to comply with the 80-character line length limit.
• Enhancement: Updated R/prior.R to use cli::cli_abort() and cli::cli_warn() in the .check_latent_priors helper function. This replaces base R stop() and warning() calls, aligning with the project's use of the cli package for user interaction.

The changes ensure that the latent model prior validation is both syntactically correct (passing linter checks) and stylistically consistent (using cli).

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PR created automatically by Jules for task 9802271797329737346 started by @seabbs

[google-labs-jules]

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[seabbs]

LGTM