Remove return.missing baseline and fix its issue

[dpanta94]

Removes in total 41 errors.

Removes errors from the below phpstan baselines and fixes the issues that they were covering:

• return.missing

How ?

Adds return types according to method docblocks.

Trac ticket: https://core.trac.wordpress.org/ticket/65817

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Opus 5
Used for: Help with suggesting how to fix the phpstan output for the specific errors we are removing the baselines for. The actual result has been reviewed and are owned by me.

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[dmsnell]

With a number of mixed categories of missing returns, this PR is probably too big to merge as a single change. Could we split it into those which should be benign by simply adding a missing return null; as the last statement in a function, from those which involve Docblock changes, from those which rearrange the return code (e.g. general-template.php, with the $display check).

those last ones seem trivial, but I fear from experience that it’s easy to accidentally flip a boolean and then introduce a regression