Increase test coverage in `keras_core/backend`

[Faisal-Alsrheed]

This PR aaims to enhance the test coverage in the keras_core/backend module.

Highlights:

1- Introduced comprehensive unit tests in keras_core/backend/common/keras_tensor_test.py to ensure the proper functioning and error handling of various tensor-related operations, such as:

• Boolean conversion of tensors.
• Iterating over tensors.
• Various symbolic tensor operations including addition, multiplication, subtraction, and more.

2- Augmented tests in keras_core/backend/common/stateless_scope_test.py to validate:

• Error handling for invalid keys and values in state mapping.

3- Expanded tests in keras_core/backend/common/variables_test.py:

• Introduced tests for variable name validation, ensuring names are strings and don't contain certain characters.
• Ensured proper error handling when using callable initializers without specifying shape.
• Validated standardization of shapes with various edge cases, including negative dimensions.
• Added a specific test for the AutocastScope class to ensure it raises a ValueError when given a non-floating-point dtype.

[codecov[bot]]

Codecov Report

Patch coverage has no change and project coverage change: +0.09% :tada:

Comparison is base (4b7ede4) 76.21% compared to head (3be1790) 76.31%. Report is 2 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #864 +/- ## ========================================== + Coverage 76.21% 76.31% +0.09% ========================================== Files 329 329 Lines 31270 31270 Branches 6083 6083 ========================================== + Hits 23833 23864 +31 + Misses 5844 5820 -24 + Partials 1593 1586 -7 ``` | [Flag](https://app.codecov.io/gh/keras-team/keras-core/pull/864/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=keras-team) | Coverage Δ | | |---|---|---| | [keras_core](https://app.codecov.io/gh/keras-team/keras-core/pull/864/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=keras-team) | `76.22% <ø> (+0.09%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=keras-team#carryforward-flags-in-the-pull-request-comment) to find out more. [see 3 files with indirect coverage changes](https://app.codecov.io/gh/keras-team/keras-core/pull/864/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=keras-team)

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