Allow multiple cases per fixture file

[Pedro-Muller29]

Fixes #4506.

Description

In #4506, MeGaGiGaGon flagged that the longest fixture files in tests/data/cases/ are painful to work with. preview_long_strings.py and preview_long_strings__regression.py are both 1000+ lines: input and output for any given case sit hundreds of lines apart, and when a test fails the assertion points at the end-of-file mismatch with nothing tying it back to the original case.

MeGaGiGaGon's first proposal was to split big files into folders of numbered files. JelleZijlstra pushed back on numbering ("renumbering files would be needlessly annoying") and suggested an alternative shape: sections within larger files, like mypy's test data format.

This PR takes the mypy-style path. Fixture files can group multiple cases under [case <name>] headers, and the two longest files are migrated as the first consumers.

# flags: --unstable

[case assign_x_1]
x = "..."
# output
x = (
    "..."
)

[case assign_y]
y = "Short string"
# output
y = "Short string"

The format is opt-in. Files without a [case ] header at the top run through the legacy single-case loader unchanged. Cell names appear in pytest IDs (test_simple_format[preview_long_strings::assign_x_1]), and on failure the assertion message attaches the file path, the cell header line, and the # output marker line, so the traceback points at the case under test rather than the file boundary.

A # flags: line above the first [case ] applies to every cell, and cell flags take precedence on overlap. That makes preview-to-stable migrations more granular: cells can graduate one at a time, while neighbors in the same file keep their preview flags.

Documentation in docs/contributing/the_basics.md covers the format and the merge semantics.

I picked preview_long_strings* for the first migrations because they're the worst offenders on file length.

Checklist - did you ...

• Implement any code style changes under the --preview style, following the stability policy?
• Add an entry in CHANGES.md if necessary?
• Add / update tests if necessary?
• Add new / update outdated documentation?