AI: Remove dead polyfills.php from php-ai-client

[gziolo]

Trac ticket: https://core.trac.wordpress.org/ticket/64591

Summary

• Removes src/wp-includes/php-ai-client/src/polyfills.php which is dead code — the generated autoloader only handles class autoloading, so this function-only file is never loaded
• The four polyfilled functions (array_is_list, str_starts_with, str_contains, str_ends_with) are already provided by WordPress Core in wp-includes/compat.php
• Updates tools/php-ai-client/installer.sh to strip the file during future library updates

Follow-up to #10881 (comment).

🤖 Generated with Claude Code

[github-actions]

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Props gziolo, flixos90.

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[github-actions]

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[felixarntz]

@gziolo Thank you, LGTM!

[felixarntz]

Suggested change

	# Remove polyfills.php — WordPress Core already provides these functions in compat.php.
	# Remove polyfills.php — WordPress Core already provides these functions in compat.php, and the file is never loaded by the custom autoloader.

[felixarntz]

Committed in https://core.trac.wordpress.org/changeset/61786