Fix matrix binary search missing values at row starts - #15560

Describe your change

• Add an algorithm?
• 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.
• Documentation change?

Fix mat_bin_search() missing a target that equals the first element of a row after the first row.

For example, mat_bin_search(3, [[1, 2], [3, 4]]) previously returned [-1, -1] instead of [1, 0].

The row-selection loop used <, stopping before searching a row whose first element equaled the target. Changing the condition to <= allows that row to be searched.

This PR changes one condition in one algorithm file. Existing doctests remain unchanged.

Validation:

• Existing doctests: 8 passed, 0 failed.
• 2,922 generated search cases passed.
• Row-start searches in an interleaved sorted matrix passed.
• Ruff lint and formatting checks for the changed file passed.
• git diff --check passed.

Checklist

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• 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.
• All new Python files are placed inside an existing directory. — No new files added.
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• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing. — All 8 existing doctests pass locally.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation. — No new algorithms added.
• If this pull request resolves one or more open issues, then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER". — No linked issues to close.