Update ContentRenderer.php

[mathetos]

Fixes #16

When the HTML-to-Markdown converter throws, the code now falls back to wp_strip_all_tags() plus html_entity_decode() instead of causing a fatal error.

Context

When a post or page contains HTML that causes the League HTML-to-Markdown converter to throw (e.g., malformed or edge-case markup), the plugin previously did not catch the exception. This resulted in a PHP fatal error and a 500 response instead of returning markdown or a safe fallback. This PR adds a try/catch around the conversion call and uses a plain-text fallback so .md URLs always return content instead of failing.

Summary

This PR can be summarized in the following changelog entry:

• Bug fix: Gracefully handle HTML-to-Markdown conversion failures. When the League converter throws, fall back to wp_strip_all_tags() and html_entity_decode() instead of causing a fatal error. Ensures .md URLs always return output.

Relevant technical choices:

• Try/catch around convert(): Wraps the converter call in a \Throwable catch so both Exception and Error are handled.
• Fallback behavior: Uses wp_strip_all_tags() for the HTML → plain text fallback and html_entity_decode() to decode entities, mirroring the behavior used for code blocks elsewhere in the renderer.
• No filter added: This PR only adds the bug fix. A separate PR or issue can add a filter for customizing the fallback if desired.

Test instructions

1. Create or edit a post with content that can trigger a converter exception (e.g., unusual or malformed HTML, or content known to break the converter).
2. Visit the post’s .md URL (e.g. https://yoursite.com/post-slug.md).
3. Expected: The page returns markdown or plain text instead of a 500 or fatal error.
4. Confirm normal posts still convert to markdown correctly.
5. Confirm code blocks and other supported content still render as expected.

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 — Use at least one post and one page to verify behavior.
• Changes should be tested on different editors (Block/Classic/Elementor/other)
• Changes should be tested on different browsers
• Changes should be tested on multisite

UI changes

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

Documentation

• I have written documentation for this change.

Quality assurance

• I have tested this code to the best of my abilities.
• I have added unit tests to verify the code works as intended.
• I have checked that the base branch is correctly set.

Fixes #16