fix(1011): default molimg height/width when not supplied

[claude-im]

Fixes #1011.

Problem

/api/molimg/ returns 500 in production. MolImageSerializer.get_mol_image sized the image from the request's height/width query parameters, but guarded that lookup with a bare if params::

params = request.query_params
if params:                                  # true if ANY parameter is present
    return draw_mol(
        obj.smiles,
        height=int(float(params["height"])),  # KeyError when height wasn't one of them
        width=int(float(params["width"])),
    )
else:
    return draw_mol(obj.smiles, height=125, width=125)

MolImgFilter accepts target, cmpd, smiles and site_observation_groups, so /api/molimg/?target=1 — or plain ?page=2 pagination — makes params truthy with no size in it, and the lookup raises MultiValueDictKeyError: 'height'. The all-or-nothing guard only ever worked for requests that supplied both dimensions or no parameters at all.

Fix

Each dimension is read through a small module-level _int_param() helper and defaults independently, so filtering and pagination parameters no longer affect sizing:

params = self.context["request"].query_params
return draw_mol(
    obj.smiles,
    height=_int_param(params, "height", self.DEFAULT_HEIGHT),
    width=_int_param(params, "width", self.DEFAULT_WIDTH),
)

The 125×125 defaults become named class attributes rather than repeated literals. Behaviour is otherwise unchanged: values are still read via float() so decimal strings are accepted and truncated.

No API contract change — responses that worked before are byte-identical.

Out of scope (noted, not fixed)

A supplied but unparseable size (?height=abc) still raises ValueError → 500. That is pre-existing behaviour and a different defect; properly it should be a 400 from request validation. Happy to file it separately if you'd like it dealt with.

Testing

New regression test viewer/tests/test_molimg_serialization.py, written first (TDD) — it reproduced the exact production MultiValueDictKeyError before the fix. It covers:

• a filtering parameter with no size → defaults (the reported 500);
• no parameters at all → defaults;
• explicit height/width → honoured;
• only one dimension supplied → the other defaults;
• float-valued sizes → truncated.

Full suite: 350 passed, 1 skipped. pre-commit (isort/black/mypy/pylint) clean.

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