Add keras3 environment in tests

[marco66colombo]

Description

• Move CI to new testing image 0.6.3.base (Py3.12, Keras3) and keep the default pytest jobs on the Keras2 env.
• Update generate_ci_yaml.py so that Keras3-only tests run in a separate Keras3 job. Other tests stay batched on Keras2 env. Keras3-only set for: test_keras_v3_api.py, test_hgq2_mha.py, test_einsum_dense.py, test_qeinsum.py.
• Keras3 coverage is split out because many legacy tests still hit Keras3 API changes; hgq2 MHA remains incompatible with Keras3 (parallelization_factor kwarg).

Note: pinned pytest<9 in testing extras to avoid pytest 9 hard error on marks applied to fixtures (existing tests still use that pattern).

Future work: decide which tests to port to Keras3 vs. keep on Keras2 vs. run on both, and track hgq2/Keras3 compatibility for MHA.

Type of change

• CI/config change
• Bug fix
• Documentation update
• New feature
• Breaking change
• Other

Tests

• CI: will run split Keras2/Keras3 jobs; no local test run.

Checklist

• I have read the contributing guidelines.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• I have installed and run pre-commit on the files I edited or added.
• I have added tests that prove my fix is effective or that my feature works.

[marco66colombo]

• Moved test_multiout_onnx into the Keras3 env/jobs because it was failing in the old (TF 2.14) env due to onnx/onnxscript/onnx-ir dependency conflicts. The newer env has compatible versions.

• Temporarily skipped test_hgq2_mha since the current hgq2 release rejects the parallelization_factor kwarg that hls4ml passes.