Let custom item types behave as starting materials or equipment

[jbouquiaux]

Follow up on #2204

Plugins can already define custom item types that subclass StartingMaterial or Equipment, but in practice those types still behave like samples: they show up on the Samples page and are private to whoever created them. That's because the core decides listings and permissions from type strings ("starting_materials", "equipment"), so a subclass with its own type never matches.

This PR lets a plugin opt in to the behaviour of the built-in type it extends:

class Precursor(StartingMaterial):
    model_config = ConfigDict(json_schema_extra={"datalab_behave_as": "starting_materials"})
    type: Literal["precursors"] = "precursors"

With the hint, Precursor items are listed and created from the Starting Materials page, are shared like inventory (no owner, visible to everyone or restricted to groups, following UNGROUPED_INVENTORY), and show up in item pickers that search for starting materials. Equipment works the same way.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.53%. Comparing base (ddb057c) to head (29dede4).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2205      +/-   ##
==========================================
+ Coverage   82.48%   82.53%   +0.05%     
==========================================
  Files          91       91              
  Lines        8782     8797      +15     
==========================================
+ Hits         7244     7261      +17     
+ Misses       1538     1536       -2     

Files with missing lines	Coverage Δ
pydatalab/src/pydatalab/models/__init__.py	96.29% <100.00%> (+0.42%)	⬆️
pydatalab/src/pydatalab/models/schema_hints.py	90.72% <100.00%> (+2.42%)	⬆️
pydatalab/src/pydatalab/permissions.py	92.89% <100.00%> (ø)
pydatalab/src/pydatalab/routes/v0_1/info.py	84.96% <ø> (ø)
pydatalab/src/pydatalab/routes/v0_1/items.py	86.84% <100.00%> (+0.01%)	⬆️

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