Taxonomy: Return a descriptive WP_Error when a term name exceeds the DB column length

[irozum]

wp_insert_term() and wp_update_term() never validate the length of the term name before writing it, unlike the equivalent checks already in place for wp_insert_user(), wp_check_comment_data_max_lengths(), register_post_type(), and register_taxonomy(). When a sanitized name exceeds the wp_terms.name column's length, the create path fails with a generic db_insert_error ("Could not insert term into the database.") instead of a descriptive error, and the update path is worse: $wpdb->update()'s return value is never checked, so wp_update_term() reports success while silently discarding the change.

This adds a small private helper, _wp_check_term_name_length(), that reads the actual column length via $wpdb->get_col_length( $wpdb->terms, 'name' ) (falling back to the schema default of 200) and compares it against the name's character count with mb_strlen() — matching the DB column's char-typed length semantics rather than counting bytes. Both wp_insert_term() and wp_update_term() now call this right after their existing empty-name checks and return a new term_name_too_long WP_Error before attempting any database write.

The original ticket's reported symptom (multibyte category/tag names getting silently truncated to invalid UTF-8 and losing all their bytes) no longer reproduces on current trunk — strip_invalid_text() now truncates on character boundaries via mb_substr() for char-typed columns, so wp_check_invalid_utf8() never sees a split sequence. What remains, and what this PR fixes, is the missing length validation itself: crossing the limit no longer causes silent data loss, but it still surfaced as an unhelpful low-level DB error (or, on update, as a false "success").

The separate bug where wp_update_term() reports success without saving any change (unrelated to name length) is filed as its own ticket, #66007, and is intentionally out of scope here.

Trac ticket: https://core.trac.wordpress.org/ticket/36610

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Used for: Implementation, tests, and PR description. Reviewed by irozum.

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