Taxonomy: Avoid repeatedly decoding the term hierarchy.

[sekalan]

Load the hierarchy once for descendant traversal and reuse it within the hierarchical hide_empty pass. Preserve descendant ordering and visibility checks, with regression coverage for hierarchy reads and term changes.

Trac ticket: https://core.trac.wordpress.org/ticket/66242

Use of AI Tools

AI assistance: Yes
Tool(s): Codex, Gemini Web
Model(s): GPT-6.1, Sol, Gemini 3.5 Flash
Used for: Initial performance investigation, code skeleton, tests

This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

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Core Committers: Use this line as a base for the props when committing in SVN:

Props serhatsoylu06, westonruter.

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[github-actions]

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[westonruter]

@sekalan I think you referenced the wrong Trac ticket for this. You opened #13986 for Core-66241 already.

[westonruter]

I've corrected the reference to Core-66242

[westonruter]

@sekalan Please include the AI Disclosure in the PR description:

wordpress-develop/.github/pull_request_template.md

Lines 22 to 33 in 794e479

	## Use of AI Tools
	<!--
	You are free to use artificial intelligence (AI) tooling to contribute, but you must disclose what tooling you are using and to what extent a pull request has been authored by AI. It is your responsibility to review and take responsibility for what AI generates. See the WordPress AI Guidelines: <https://make.wordpress.org/ai/handbook/ai-guidelines/>.
	Example disclosure:
	AI assistance: Yes
	Tool(s): GitHub Copilot, ChatGPT
	Model(s): GPT-5.1
	Used for: Initial code skeleton and test suggestions; final implementation and tests were reviewed and edited by me.
	-->

[sekalan]

@sekalan Please include the AI Disclosure in the PR description:

wordpress-develop/.github/pull_request_template.md

Lines 22 to 33 in 794e479

	## Use of AI Tools
	<!--
	You are free to use artificial intelligence (AI) tooling to contribute, but you must disclose what tooling you are using and to what extent a pull request has been authored by AI. It is your responsibility to review and take responsibility for what AI generates. See the WordPress AI Guidelines: <https://make.wordpress.org/ai/handbook/ai-guidelines/>.
	Example disclosure:
	AI assistance: Yes
	Tool(s): GitHub Copilot, ChatGPT
	Model(s): GPT-5.1
	Used for: Initial code skeleton and test suggestions; final implementation and tests were reviewed and edited by me.
	-->

Thanks @westonruter, sorry for the confusion with the ticket reference. i started investigating this second issue using gpt, but hit my usage limit right before completing it, so i switched to gemini 3.5 flash to finish the task and submit the pr. that's why i mixed up the ticket number and was unsure about how to format the disclosure. i appreciate you fixing the reference! I've also updated the pr description with the ai disclosure.