Fix exponential_search recursing forever when the item is below the first element

[Darkslayer3324j]

Describe your change

• 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.

exponential_search (and binary_search_by_recursion) never terminate when the item is smaller than every element:

>>> exponential_search([0, 5, 7, 10, 15], -3)
RecursionError: maximum recursion depth exceeded
>>> binary_search_by_recursion([0, 5, 7, 10, 15], -1)
RecursionError: maximum recursion depth exceeded

binary_search_by_recursion uses right=-1 to mean "not given" (if right < 0: right = len(...) - 1). When the item is below the first element the recursion legitimately reaches right = midpoint - 1 = -1, which is then mistaken for "not given", right is reset to the last index, and the search starts over forever. The fix makes the sentinel None, which cannot collide with a real index. Nothing else in the repository calls this function.

Doctests for an item below and above the range are added. I also compared both functions with a membership check on 30,000 random sorted lists (lengths 0-15, values -8..8): 0 mismatches after the change.

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.
• All filenames are in all lowercase characters with no spaces or dashes.
• 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 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: "Fixes #ISSUE-NUMBER". (No open issue for this.)

I left "all my own work" unticked on purpose: this change was written with AI assistance (Claude Code, as AGENTS.md invites). I found the problem by fuzzing the functions in searches/ against a membership check, reproduced it, and ran the doctests, ruff check and ruff format on the changed file.

[oga35767-eng]

👍