estimate_contrasts() gets a post_process argument

[strengejacke]

No description provided.

[gemini-code-assist]

Code Review

This pull request introduces a new post_process argument to estimate_contrasts() and related functions to support multi-step comparisons and post-processing of results. It also updates documentation, adds corresponding tests, and makes minor adjustments to helper functions. However, two issues were identified in the review: first, a runtime error will occur in .guess_marginaltrends_arguments() because the variable dots is undefined; second, the newly added tests are unnecessarily skipped for R versions below 4.5.0, which would prevent them from running on current stable releases of R.

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