Fix radix_tree.py insertion fail in ["*X", "*XX"] cases

[furthermares]

Fixes: #8888

Describe your change:

Consider a word, and a copy of that word, but with the last letter repeated twice. (e.g., ["ABC", "ABCC"])
When adding the second word's last letter, it only compares the previous word's prefix—the last letter of the word already in the Radix Tree: 'C'—and the letter to be added—the last letter of the word we're currently adding: 'C'. So it wrongly passes the "Case 1" check, marks the current node as a leaf node when it already was, then returns when there's still one more letter to add.
The issue arises because prefix includes the letters of the node itself. (e.g., nodes: {'C' : RadixNode()}, is_leaf: True, prefix: 'C') It can be easily circumvented by simply adding the is_leaf check, as the code already assumes you only input new words.

• Test Case: "A AA AAA AAAA AAAAA AAAAAA"
  • Fixed correct output:

  Words:['A', 'AA', 'AAA', 'AAAA', 'AAAAA', 'AAAAAA']
  Tree:
  - A   (leaf)
  -- A   (leaf)
  --- A   (leaf)
  ---- A   (leaf)
  ----- A   (leaf)
  ------ A   (leaf)
  

  • Current incorrect output:

  Words: ['A', 'AA', 'AAA', 'AAAA', 'AAAAA', 'AAAAAA']
  Tree:
  - A   (leaf)
  -- AA   (leaf)
  --- A   (leaf)
  ---- AA   (leaf)
  

1. ✅ Insert "A". Node is empty. Insert "A". Node prefix = "A".
2. ❌ Insert "AA". Compare node prefix "A" to "AA". "A" == "A". Fails to insert. No change.
3. ✅ Insert "AAA". Compare node prefix "A" to "AAA". "A" != "AA". Insert "AAA". Node prefix = "AA".
4. ✅ Insert "AAAA". Compare node prefix "AA" to "AAAA". "AA" != "A". Case 2: Since "AAA" exists, branch and insert "AAAA". Node prefix = "A". Follows the Step 1 pattern.
5. ❌ Insert "AAAAA". Follows the Step 2 pattern.
6. ✅ Insert "AAAAAA". Follows the Step 3 pattern. (And the next step would follow Step 1's.)

N.B. It passed test cases for Croatian Open Competition in Informatics 2012/2013 Contest #3 Task 5 HERKABE

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?

Checklist:

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[cclauss]

Nice work! Thanks for doing this.