[Pixi] Build SofaPython3 against pybind11 < 3.1

[brentDSL]

pixi.lock currently resolves pybind11 3.1.0 for every environment. pybind11 3.1.0 runs keep_alive's postcall on the try-next-overload sentinel, so a binding declared with keep_alive<0, N> segfaults when called with arguments that match no overload, instead of raising TypeError. In SofaPython3 that is Node.addObject:

root = Sofa.Core.Node("root")
root.addObject(1.0)   # pybind11 3.0.x: TypeError -- pybind11 3.1.0: segfault

It also makes Bindings.Sofa.Tests crash at Simulation/Node.py::test_createObjectInvalid. It is fixed upstream by pybind/pybind11#6183, not yet released.

This PR constrains the plugin-sofa-python3 feature to pybind11 >=2.3.0,<3.1 and relocks. The pixi.lock change is only pybind11 3.1.0 -> 3.0.4 (no other package moves); it was generated with pixi 0.67.2 so the lock file stays at format v6.

The crash does not depend on the Python version (reproduced on 3.12, 3.13 and 3.14). Tested on linux-64, SOFA master + SofaPython3 master built in a pixi environment on Python 3.14.7, with SofaPython3 tests enabled:

	pybind11 3.1.0	pybind11 3.0.4
addObject(1.0)	segfault	TypeError
Bindings.Sofa.Tests	segfault at test 107	148/148
Bindings.Modules.Tests	-	11/11
Bindings.SofaRuntime.Tests	2/2	2/2
Bindings.SofaTypes.Tests	6/6	6/6

The upper bound can be lifted once a pybind11 release includes #6183. The same pin went into conda-forge's sofa-python3 build (conda-forge/sofa-python3-feedstock#4).

[with-all-tests]

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• it builds with SUCCESS for all platforms on the CI.
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• it does not break API compatibility.
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[bakpaul]

Superseded by #6366