Build/QA: Retire identical.alwaysTrue PHPStan baseline

[huzaifaalmesbah]

Description

Retires the identical.alwaysTrue PHPStan baseline in WordPress core by removing a redundant comparison and adding a targeted type annotation:

1. File_Upload_Upgrader: In the plugin/theme archive check, because 'pluginzip' was already matched above, the remaining branch is always 'themezip'. Simplified if ( 'themezip' === $form ) to else.
2. rest_filter_response_fields(): Annotated $fields_as_keyed as array<string, mixed> to clarify the nested array structure to PHPStan without redundant strict comparison warnings.

The always-true '404' === $error check in WP::parse_request() was already removed in r64244, so these are the last two errors and identical.alwaysTrue.neon is retired.

Testing Instructions

1. Run static analysis:

   composer phpstan

   Confirm 0 errors are found.

2. Run PHPCS:

   vendor/bin/phpcs src/wp-admin/includes/class-file-upload-upgrader.php src/wp-includes/rest-api.php

   Confirm standards compliance.

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

Use of AI Tools

AI assistance: Yes
Tool(s): Google Antigravity
Model(s): Gemini 3.8 Flash
Used for: Researching static analysis reports and drafting initial baseline diffs; all logic, type declarations, and analysis results were manually investigated, verified, and run locally by me.

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Props huzaifaalmesbah.

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[github-actions]

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