Python bindings: fix use-after-free on field definitions

[alonfaraj]

What does this PR do?

GetLayerDefn() and GetDefnRef() already hold a referene to the FeatureDefn, but the field defns you get out of it don't, so once the dataset or feature goes away they point at freed memory:

from osgeo import ogr
ogr.UseExceptions()

def first_field(path):
    ds = ogr.Open(path)
    return ds.GetLayer(0).GetLayerDefn().GetFieldDefn(0)

print(first_field("poly.shp").GetName())   # garbage instead of 'AREA'

Same thing with GetGeomFieldDefn(0) - segfaults when calling GetSpatialRef().

Reproduced 3.13.3 and master.

Fix

Make returned objects keep a Python reference to their parent:

• FeatureDefn.GetFieldDefn / GetGeomFieldDefn keep FeatureDefn
• Feature.GetFieldDefnRef / GetGeomFieldDefnRef keep Feature
• GeomFieldDefn.GetCoordinatePrecision keep GeomFieldDefn

Tests

Added test_field_defn_use_after_layer_del and test_field_defn_use_after_feature_del

What are related issues/pull requests?

AI tool usage

• AI (Y-a-t-il-un-Copilot-dans-l'avion, Chat-j'ai-pété, Jean-Claude Dusse or something similar) supported my development of this PR. See our policy about AI tool use. Use of AI tools must be indicated.

Tasklist

• Make sure code is correctly formatted (cf pre-commit configuration)
• Add test case(s)
• Add documentation
• Updated Python API documentation (swig/include/python/docs/)
• Review
• Adjust for comments
• All CI builds and checks have passed
• ADD YOUR TASKS HERE

Environment

Provide environment details, if relevant:

• OS: Ubuntu 26.04
• Compiler: gcc (Ubuntu 15.2.0-16ubuntu1) 15.2.0