… inside Case 3

Describe your change: Fixes #11316

The insert() method in radix_tree.py contained a Case 1 branch that was
practically unreachable due to how recursion is structured.

Exact matches are detected during prefix matching (Case 3),
so handling the exact-match case there removes redundancy
and simplifies the logic.

This change removes the unreachable Case 1 branch
and handles exact matches directly inside Case 3
without altering existing behavior.

• 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.
• Documentation 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".