Fix ty invalid assignment

[kadubhumika]

Describe your change:

Fixed remaining ty invalid-assignment and related type-checking diagnostics across multiple existing files.

Changes include:

• Fixed type annotations and optional value handling.
• Added necessary assertions for image pixel access.
• Fixed typing issues in linked lists, segment trees, heaps, automatic differentiation, and minimum cut implementations.
• Explicitly imported urllib.request.
• Fixed the CPU scheduling selection type-checking issue.
• Updated DIRECTORY.md.

Validation performed:

ty check --exclude-scripts
All checks passed!



### Checklist:
* [ ] Add an algorithm?
* [x] Fix a bug or typo in an existing algorithm?
* [ ] Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
* [x] Documentation change?
* [x] I have read [CONTRIBUTING.md](https://github.com/TheAlgorithms/Python/blob/master/CONTRIBUTING.md).
* [x] This pull request is all my own work -- I have not plagiarized.
* [x] I know that pull requests will not be merged if they fail the automated tests.
* [ ] This PR only changes one algorithm file.  To ease review, please open separate PRs for separate algorithms.
* [x] All new Python files are placed inside an existing directory.
* [x] All filenames are in all lowercase characters with no spaces or dashes.
* [x] All functions and variable names follow Python naming conventions.
* [x] All function parameters and return values are annotated with Python [type hints](https://docs.python.org/3/library/typing.html).
* [ ] All functions have [doctests](https://docs.python.org/3/library/doctest.html) that pass the automated testing.
* [ ] All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
* [ ] If this pull request resolves one or more open issues then the description above includes the issue number(s) with a [closing keyword](https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue): "Fixes #ISSUE-NUMBER".