Posts, Taxonomy: Skip wp_set_post_categories() on update when post_category is not provided

[wppoland]

Trac ticket: https://core.trac.wordpress.org/ticket/59354

wp_insert_post() always called wp_set_post_categories() on update, even when the caller did not pass post_category. The fallback then re-set the post's existing category list, which still goes through term-cache invalidation, term-relationship lookups, and counted toward the per-request query budget - a measurable regression for callers like REST API updates that change a single field.

This PR gates that call on either (a) it being a new post or (b) post_category being explicitly set in the args. The tags_input and tax_input paths were already correctly guarded.

Tests cover:

• set_object_terms does not fire for the category taxonomy on a parameterless update
• Existing categories survive a parameterless update (regression guard)

Use of AI Tools

AI assistance: Yes
Tool(s): Antigravity IDE
Model(s): Gemini 2.5 Pro
Used for: Code analysis, conflict resolution, test suite updates, and patch verification. All changes were manually reviewed and tested locally.

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