chore: add orjson to dependencies

[himkt]

Summary

orjson is installed via the indirect dependency of pymatgen (since v2025.5.28 materialsproject/pymatgen#4411).

However, current mp-api allows users to use the older versions of pymatgen, which do not depend on orjson, which causes a runtime error after installation.

api/pyproject.toml

Line 24 in 7d50936

"pymatgen>=2022.3.7,!=2024.2.20",

Given the situation, I think we have two options: (1) increasing the minimum version of pymatgen or (2) add orjson to dependency of mp-api. I think it would be better to adopt (2) since orjson is actually used in the mp-api codebase (even if installing orjson by increasing the minimum version of pymatgen, that is indirectly installed. I think the dependency should be declared explicitly).

---

How to replicate the issue

This problem happens when third-party package depends on the specific version of pymatgen, and any version of mp-api.

# use old pymatgen (e.g. third-party package depends on the specific version of pymatgen, and any version of mp-api)
> uv pip install 'pymatgen<v2025.4.16' mp-api
Resolved 56 packages in 531ms
      Built pymatgen==2025.4.10
Prepared 1 package in 12.19s
Installed 56 packages in 368ms
 + annotated-types==0.7.0
 + blake3==1.0.8
 + boto3==1.40.52
 + botocore==1.40.52
 + certifi==2025.10.5
 ...
 + spglib==2.6.0
 + sympy==1.14.0
 + tabulate==0.9.0
 + tqdm==4.67.1
 + typing-extensions==4.15.0
 + typing-inspection==0.4.2
 + tzdata==2025.2
 + uncertainties==3.2.3
 + urllib3==2.5.0
 + wrapt==1.17.3

> uv run python -c 'from mp_api.client import MPRester'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
    from mp_api.client import MPRester
  File "/Users/himkt/Desktop/tmp-proj/.venv/lib/python3.13/site-packages/mp_api/client/__init__.py", line 7, in <module>
    from .core import MPRestError
  File "/Users/himkt/Desktop/tmp-proj/.venv/lib/python3.13/site-packages/mp_api/client/core/__init__.py", line 3, in <module>
    from .client import BaseRester, MPRestError, MPRestWarning
  File "/Users/himkt/Desktop/tmp-proj/.venv/lib/python3.13/site-packages/mp_api/client/core/client.py", line 41, in <module>
    from mp_api.client.core.utils import load_json, validate_ids
  File "/Users/himkt/Desktop/tmp-proj/.venv/lib/python3.13/site-packages/mp_api/client/core/utils.py", line 6, in <module>
    import orjson
ModuleNotFoundError: No module named 'orjson'

[tschaume]

@himkt good catch and thank you for your contribution! Release candidate v0.45.13rc2 is available on PyPI now.