Fix code block whitespace handling in Markdown

[yalagadapavankumar]

Fix Markdown code block closing whitespace handling

Previously, the parser incorrectly accepted closing backticks followed by extra text and rejected lines where only spaces appeared after closing backticks. This did not match expected Markdown behavior.

Now, the parser correctly ends a code block only when line after the code ends with spaces or nothing. Lines where extra text appears after the closing backticks are treated as part of the code block.

Includes tests for both correct and incorrect cases.

Related issue

This PR addresses the Outreachy issue: Markdown whitespace bug in Rust Compiler

[rustbot]

r? @JohnTitor

rustbot has assigned @JohnTitor.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: compiler
• compiler expanded to 69 candidates
• Random selection from 12 candidates