Codex/assertion text diff blocks

[hamza-mobeen]

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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• Create a new changelog file in the changelog directory, with a name like <ISSUE NUMBER>.<TYPE>.rst.
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[hamza-mobeen]

@RonnyPfannschmidt I'm still waiting on your feedback :)

[RonnyPfannschmidt]

This. Technically looks good but is also at odds with a number of comments on the original issue

As such id like to defer to @nicoddemus

[nicoddemus]

Thanks @hamza-mobeen for the PR, overall great work!

I left some comments, please take a look.

[nicoddemus]

Let's clearly present the two options:

Suggested change

	:confval:`assertion_text_diff_style`: Controls how pytest renders ``str == str`` failures. The default ``ndiff`` output
	keeps the existing inline diff markers. Setting it to ``block`` prints multiline string comparisons as separate
	``Left:`` and ``Right:`` blocks, which can be easier to read when whitespace or indentation differences dominate.
	:confval:`assertion_text_diff_style`: Controls how pytest renders ``str == str`` failures.
	* ``ndiff`` (the default) outputs the differences using inline diff markers.
	* ``block`` prints multiline string comparisons as separate ``Left:`` and ``Right:`` blocks, which can be easier to read when whitespace or indentation differences dominate.
	Note that it is possible to set this option (as any other configuration option) directly in the command line using ``-o assertion_text_diff_style=block``.

[nicoddemus]

Suggested change

	* ``ndiff``: use the default inline diff rendering.
	* ``ndiff``: use the inline diff rendering markers.

[nicoddemus]

Suggested change

	* ``block``: render multiline string comparisons as separate ``Left:`` and ``Right:`` blocks.
	* ``block``: render each string in separate ``Left:`` and ``Right:`` blocks.

[nicoddemus]

Why is this necessary? Is the config being passed here different from the one already in util._config?

[nicoddemus]

Also, given this is the only place where util.assertrepr_compare is called, consider just passing the text diff style directly to util.assertrepr_compare.

[nicoddemus]

Lets remove this default value and force callers to always pass it; it is an internal function, so breaking the API is actually a good thing because it will ensure every caller is actually passing the right thing.

Suggested change

	assertion_text_diff_style: str = ASSERTION_TEXT_DIFF_STYLE_NDIFF,
	assertion_text_diff_style: str,

[nicoddemus]

Not sure we should special case being multiline text or not; I think it is reasonable to always honor the block configuration, even for single line texts.

[nicoddemus]

Instead of typing this as str, let's use Literal["ndiff", "block"]. This way we are more explicit, plus we can use match below.