Fix dulwich checkout_branch deprecation warning in testing utilities

[Copilot]

Why

Integration tests emit deprecation warnings from dulwich 0.22.9:

DeprecationWarning: checkout_branch is deprecated since 0.22.9 and will be removed in 0.24.0

Goal: eliminate warning by migrating to current dulwich API.

What changed

• Replace porcelain.checkout_branch(repo, branch.encode("utf-8")) with porcelain.checkout(repo=repo, target=branch) in infrahub_sdk/testing/repository.py
• No behavior change: checkout() performs identical operation with cleaner signature (handles string/bytes transparently)

How to review

Single-line change in GitRepo.init(). Old API took encoded bytes; new API accepts strings directly.

How to test

Run integration test that triggered the warning:

pytest tests/integration/test_repository.py::TestInfrahubRepository::test_add_repository -v

No deprecation warning should appear.

Impact & rollout

• Backward compatibility: None - internal testing utility only
• Performance: No change
• Deployment notes: Safe to deploy

Checklist

• Tests added/updated
• Changelog entry added (uv run towncrier create ...)
• External docs updated (if user-facing or ops-facing change)
• Internal .md docs updated (internal knowledge and AI code tools knowledge)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>task: Deprecation warning in CI with branch checkout</issue_title>
<issue_description>### Component

No response

Task Description

Running the integration tests shows this warning:

tests/integration/test_repository.py::TestInfrahubRepository::test_add_repository
  /var/lib/github/ghrunner_2/infrahub-sdk-python/infrahub-sdk-python/infrahub_sdk/testing/repository.py:76: DeprecationWarning: checkout_branch is deprecated since 0.22.9 and will be removed in 0.24.0
    porcelain.checkout_branch(self._repo.git, self.initial_branch.encode("utf-8"))
```</issue_description>

<agent_instructions>Review what changes needs to be done based on the code within the third party package. </agent_instructions>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes task: Deprecation warning in CI with branch checkout #818

---

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View logs

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@            Coverage Diff             @@
##           stable     #819      +/-   ##
==========================================
- Coverage   80.37%   80.34%   -0.04%     
==========================================
  Files         115      115              
  Lines        9873     9873              
  Branches     1504     1504              
==========================================
- Hits         7935     7932       -3     
- Misses       1416     1420       +4     
+ Partials      522      521       -1     

Flag	Coverage Δ
integration-tests	41.41% <100.00%> (-0.04%)	⬇️
python-3.10	51.40% <100.00%> (ø)
python-3.11	51.40% <100.00%> (ø)
python-3.12	51.38% <100.00%> (-0.03%)	⬇️
python-3.13	51.40% <100.00%> (ø)
python-3.14	53.03% <100.00%> (-0.03%)	⬇️
python-filler-3.12	24.06% <0.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
infrahub_sdk/testing/repository.py	77.77% <100.00%> (ø)

... and 1 file with indirect coverage changes

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