Fixes processing of hyphens at start and end positions

[mhkuu]

Context

• We want to fix a bug where using hyphens at the start or end of a sentence would cripple the content analysis.

Summary

This PR can be summarized in the following changelog entry:

• Fixes a bug where using hyphens at the start or end of a sentence would cripple the content analysis when editing a post.

Relevant technical choices:

• None.

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

• Create a new post.
• Use the steps from Text contains 0 words or Not enough content when paragraph leads with a dash #21994.
• Confirm the bug is no longer reproducible, and the analysis works properly.
• Also confirm that adding multiple hyphens at the start or the end of a paragraph does not resurface the bug.
• Confirm this in multiple editors.

Relevant test scenarios

• Changes should be tested with the browser console open
• Changes should be tested on different posts/pages/taxonomies/custom post types/custom taxonomies
• Changes should be tested on different editors (Default Block/Gutenberg/Classic/Elementor/other)
• Changes should be tested on different browsers
• Changes should be tested on multisite

Test instructions for QA when the code is in the RC

• QA should use the same steps as above.

QA can test this PR by following these steps:

Impact check

This PR affects the following parts of the plugin, which may require extra testing:

• This change primarily effects the sentence length assessment, but it's an edge case.

UI changes

• This PR changes the UI in the plugin. I have added the 'UI change' label to this PR.

Other environments

• This PR also affects Shopify. I have added a changelog entry starting with [shopify-seo], added test instructions for Shopify and attached the Shopify label to this PR.

Documentation

• I have written documentation for this change. For example, comments in the Relevant technical choices, comments in the code, documentation on Confluence / shared Google Drive / Yoast developer portal, or other.

Quality assurance

• I have tested this code to the best of my abilities.
• During testing, I had activated all plugins that Yoast SEO provides integrations for.
• I have added unit tests to verify the code works as intended.
• If any part of the code is behind a feature flag, my test instructions also cover cases where the feature flag is switched off.
• I have written this PR in accordance with my team's definition of done.
• I have checked that the base branch is correctly set.

Innovation

• No innovation project is applicable for this PR.
• This PR falls under an innovation project. I have attached the innovation label.
• I have added my hours to the WBSO document.

Fixes #21994

[coveralls]

Pull Request Test Coverage Report for Build e69bcd469d808d5290bce9b76fa3ea1da3fea87c

Details

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.005%) to 54.606%

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Totals
Change from base Build 0031b1b85c60d127dcd458cbd89a54d3099c9afe:	0.005%
Covered Lines:	30086
Relevant Lines:	55466

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💛 - Coveralls

[FAMarfuaty]

Change looks good! Clean solution ✨

[FAMarfuaty]

Acceptance test: works as expected. Additionally, I also tested the AI Optimize for sentence length assessment and it worked nicely 🎉