[PyOV] Fix creation of Constant from numpy string arrays

[Iyanuoluwa007]

Details:

• Creating a Constant from a numpy str_/bytes_ array passed the fixed-width numpy buffer to the string Constant constructor as if it held std::string objects. Depending on the data this crashed the interpreter or raised MemoryError: bad allocation.
• Convert string arrays into a string Tensor first, using the same helper the Tensor constructor already uses, and build the Constant from it.
• Support non-contiguous string arrays in as_contiguous instead of throwing, and raise a clear error when shared memory is requested for strings (string data can only be copied), matching Tensor.
• Add tests for str and bytes arrays, scalar, 2D, empty string, non-contiguous input, inference with a string constant, and the shared memory error.
• The issue is currently assigned with no linked PR and no activity for several months. I asked in the issue whether it can be reassigned; if the assignees are still working on it, I am happy to close this PR.

Tickets:

• Closes [Good First Issue][Python API]: Create constant for string tensor and fix segfault #23611

AI Assistance:

• AI assistance used: yes
• The root cause analysis, fix and tests were done with an AI coding assistant.
• Validation performed: local Windows build of pyopenvino (MSVC, Release); the new tests crashed the interpreter before the change and pass after it (8 passed); test_constant.py otherwise unchanged apart from tests that need the AUTO plugin, which was not part of the local build; test_tensor.py and test_tensor_string.py: 636 passed; clang-format 18 and flake8 clean.

[Somay-kousis]

Checked the master behaviour on macOS arm64 with the 2026.5.0 nightly wheel. There it doesn't crash or raise. Every string just comes back empty, so on some platforms this is a silent wrong value:

call	master
ops.constant(np.array(["openvino"], dtype=str)).get_value_strings()	['']
ops.constant(np.array(["text", "", "x" * 40], dtype=str))	['', '', '']
ops.constant(np.array([b"bytes", b"b"], dtype=np.bytes_))	['', '']
ops.constant(np.array("scalar", dtype=str))	['']

Constant(Tensor(np.array(["a", "bc"]))) already gives ['a', 'bc'], so routing through the string Tensor helper matches what that path does.

One behaviour change outside Constant: as_contiguous is also used by Tensor's create_copied (common.cpp, the !is_contiguous branch before fill_string_tensor_data). On master, Tensor(np.array(["openvino", "test"], dtype=str)[::-1]) raises RuntimeError: Tensor cannot be created as contiguous!. With the new case ov::element::string it should copy instead. That looks like the right behaviour, but only the Constant side is tested. A case in test_tensor_string.py for a reversed or strided str_ and bytes_ array would cover the Tensor change too. I didn't build the branch, so the after-state for Tensor is inferred from the diff.