Fix Plot crashing when given a MultiFit inside a list (fixes #246)

[cverstege]

Summary

• k2.plot(multi_fit) crashed with TypeError: 'NoneType' object is not callable
  (see wrapped plotting function does not work with multifit #246). Root cause: the plot() wrapper always wraps its argument in a list before
  constructing Plot(...). Since a MultiFit isn't iterable, plot(multi_fit) builds
  Plot([multi_fit]) — and Plot.__init__ only special-cased a MultiFit passed directly
  (isinstance(fit_objects, MultiFit)), so a MultiFit hidden inside a list fell through
  to the regular per-fit path, where MultiFit.PLOT_ADAPTER_TYPE is None, causing the crash.
• Plot.__init__ (kafe2/fit/_base/plot.py) now also unwraps a lone MultiFit found inside
  a sequence, the same way it already unwraps one passed directly. So both Plot(multi_fit)
  and Plot([multi_fit]) (and therefore k2.plot(multi_fit)) work correctly.
• If a MultiFit is mixed with other fits in the same sequence (e.g.
  Plot([multi_fit, other_fit])) — a case that isn't supported today, since Plot can only
  track a single combined "global" fit-info block — this now raises a clear
  NotImplementedError instead of the confusing TypeError from before.
• Added a TestMultiFitPlot test class in kafe2/test/fit/test_plot.py covering all of the
  above (direct MultiFit, MultiFit in a list, the plot() wrapper, and the mixed-list
  error case).
• Also includes an unrelated black reformat of a few files
  (kafe2/fit/_base/plot.py, several kafe2/test/fit/test_representers_*_yaml.py) needed to
  get CI's Linting job green again — black's formatting rules for multi-line
  textwrap.dedent(...) calls/string concatenations changed between the version pinned when
  those files were last touched and the version pip install currently resolves (the repo
  doesn't pin a black version). No functional changes.

Test plan

• make test (pytest + coverage/unittest run): 850 passed, 80 skipped, 0 failed.
• make lint (isort, black, flake8): clean.
• Ran all 32 scripts under examples/ with MPLBACKEND=Agg: all exit 0, including the
  examples/011_multifit/*.py scripts.
• Reproduced the exact snippet from issue wrapped plotting function does not work with multifit #246 (k2.MultiFit([...]).do_fit() then
  k2.plot(mfit)) — no longer crashes.
• Verified Plot([multi_fit, other_fit]) now raises a clear NotImplementedError instead
  of a confusing TypeError.
• CI on the branch: PyTest — success, Linting — success.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Fixes a regression/edge case where k2.plot(multi_fit) (via the wrapper) would crash when a MultiFit instance was wrapped inside a one-element sequence, by teaching Plot.__init__ to detect and unwrap that case (and to fail fast with a clear error for unsupported mixed lists).

Changes:

• Update Plot.__init__ to unwrap a lone MultiFit found inside an iterable (e.g. [multi_fit]) and raise NotImplementedError if a MultiFit is mixed with other fits.
• Add regression tests covering Plot(multi_fit), Plot([multi_fit]), the wrapper.plot(multi_fit) path, and the mixed-list error case.
• Apply formatting-only updates (Black-driven) to several YAML representer test files and plot.py string constants.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
kafe2/fit/_base/plot.py	Unwraps MultiFit when passed inside a one-element sequence; adds explicit error for unsupported mixed sequences; minor formatting changes.
kafe2/test/fit/test_plot.py	Adds regression tests ensuring MultiFit plotting works through both Plot and the wrapper plot() helper.
kafe2/test/fit/test_representers_parametric_model_yaml.py	Formatting-only updates to multiline YAML string concatenations.
kafe2/test/fit/test_representers_model_function_yaml.py	Formatting-only updates to multiline YAML string concatenations (including raw strings).
kafe2/test/fit/test_representers_format_yaml.py	Formatting-only updates to multiline YAML string concatenations.
kafe2/test/fit/test_representers_fit_yaml.py	Formatting-only updates to multiline YAML string concatenations.
kafe2/test/fit/test_representers_container_yaml.py	Formatting-only updates to multiline YAML string concatenations.
kafe2/test/fit/test_representers_constraint_yaml.py	Formatting-only updates to multiline YAML string concatenations.

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