closes #6757

Description

This PR introduces a new configuration option, assertion_text_diff_style, to allow users to customize how multiline string assertion failures are displayed.

As highlighted in #6757, when pytest uses ndiff to display differences for multiline strings (like output from capsys), it can sometimes become an unreadable "soup", especially when changes involve indentation or leading/trailing whitespace. The current behavior often results in a confusing line-by-line diff that is difficult to parse.

This PR adds an optional block diff style that displays the exact contents of the "Left" and "Right" text blocks intact and sequentially, making it significantly easier to read the literal multiline strings being compared without the interspersed ndiff noise.

Key changes:

• Added assertion_text_diff_style configuration option. Defaults to ndiff to preserve backwards compatibility.
• Implemented the block diff style (_diff_text_block) for multiline string comparisons, solving the readability issue raised in assertion diffs for multiline-string can become unreadable soup #6757.
• Added fallback logic so that single-line comparisons still use ndiff even if block is selected.
• Added strict validation to ensure assertion_text_diff_style is either ndiff or block.
• Included extensive test coverage in testing/test_assertion.py verifying block formatting, fallback logic, blank line handling, and configuration validation.

Checklist

• Include documentation when adding new features.
• Include new tests or update existing tests when applicable.
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