Fix NameError in v3 from_zarr/ArraySpec.like and KeyError in model_dump

[d-v-b]

Summary

This PR fixes four verified runtime crashes in pydantic_zarr.v3 and pydantic_zarr.experimental.v3, identified in #165.

Bug 1: ArraySpec.like raises NameError when called with a spec or zarr array

ArraySpec.like in src/pydantic_zarr/v3.py did a bare isinstance(other, zarr.Array) but zarr is only imported under TYPE_CHECKING, so at runtime this crashes:

from pydantic_zarr.v3 import ArraySpec
spec = ArraySpec(...)
spec.like(spec)  # NameError: name 'zarr' is not defined

Fix: Use the sys.modules guard that GroupSpec.like already uses:

if (zarr := sys.modules.get("zarr")) and isinstance(other, zarr.Array):

Bug 2: Module-level from_zarr in v3.py raises NameError

The function body does isinstance(element, zarr.Array) with no runtime import:

from pydantic_zarr.v3 import from_zarr
import zarr
arr = zarr.open_array(...)
from_zarr(arr)  # NameError: name 'zarr' is not defined

Fix: Add import zarr inside the function body, matching v2.py.

Bug 3: Module-level from_zarr in experimental/v3.py raises NameError

Same as Bug 2 but in the experimental module.

Fix: Add import zarr inside the function body.

Bug 4: ArraySpec.model_dump(exclude={'dimension_names'}) raises KeyError

The model_dump override in both v3.py and experimental/v3.py did d["dimension_names"] unconditionally. When the field is excluded, the key is absent:

spec.model_dump(exclude={"dimension_names"})  # KeyError: 'dimension_names'

Fix: Use d.get("dimension_names") is None and d.pop("dimension_names", None).

Additional fixes

• conftest.py zarr 3.2 compatibility: zarr 3.2 renamed Structured to Struct (with different constructor signature); the test conftest crashed on import with zarr 3.2. Updated to detect whichever name is present in the dtype registry.
• mypy python_version: Bumped from "3.11" to "3.12" so mypy can parse zarr 3.2 code that uses the PEP 695 type statement.

Test plan

• test_arrayspec_like_spec_vs_spec — ArraySpec.like(spec) no longer raises NameError
• test_arrayspec_like_spec_vs_zarr_array — ArraySpec.like(zarr_array) no longer raises NameError
• test_from_zarr_array / test_from_zarr_group — module-level from_zarr no longer raises NameError
• test_model_dump_exclude_dimension_names — model_dump(exclude={'dimension_names'}) no longer raises KeyError
• Same four tests repeated for experimental.v3
• All 10 new regression tests pass; 19,435 existing tests unchanged

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