Silly difference: Backends have inversed contrasts

[strengejacke]

Fixes #645

• Tests need to be updated

[gemini-code-assist]

Code Review

This pull request changes the default value of the comparison argument in get_emcontrasts() from "pairwise" to "revpairwise" to ensure consistent contrast signs with the "marginaleffects" backend, alongside documentation updates and code reformatting. However, the reviewer noted that this change is currently overridden and ineffective because estimate_contrasts.default() still defaults to "pairwise". To fix this, the default in estimate_contrasts.default() should be set to NULL and resolved dynamically, and the corresponding tests should be updated.

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