Ticket 26649 escaped shortcodes excerpt

[dilipom13]

Fixes #26649.

This changeset prevents escaped shortcodes (e.g., [[shortcode]]) from being incorrectly expanded when generating excerpts using get_the_excerpt().

• Temporarily removes all registered shortcodes before applying the get_the_excerpt filters.
• Restores the original shortcode handlers after excerpt generation.
• Ensures escaped shortcodes display correctly without being processed.

Trac ticket: https://core.trac.wordpress.org/ticket/26649

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[github-actions]

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[t-hamano]

Thanks for the PR! Although multiple PRs have been submitted, but let me close this PR in order to prioritize #8231, which has already been approved.