challenge: brain teaser- solution for add string numbers

[AdlaAbouSteiti]

solution of add string numbers brain teaser challenge:
it takes inputs as string and return the answer of their addition.

• PR's title includes the challenge's name and language
• PR has short and clear description of the challenge
• PR has appropriate labels and milestones for easy identification
• PR it is assigned to the owner
• reviewers are assigned
• the PR contributes only one focused change
• the branch is up to date with main/master
• the code works when pulled and run locally
• all conflicts are resolved (if any)
• It is linked to an issue in the appropriate column of the project board
• feedback is addressed (if any, and if it is appropriate feedback.)

README Documentation

• The solution is documented in /src/README.md
• The markdown source is formatted
• Spelling and grammar is correct in all text
• The markdown looks correct when you preview the file
• All links and images work
• The README documents the solution's behavior, strategy(ies) and implementation(s)
• There are use cases in code blocks to illustrate the function's behavior

Python Files

• There is a module header
• There is a module docstring
• File names are in snake_case
• Test files are named test_<module_name>.py

Function Docstring

• Behavior description
• Parameter description
• Return value description
• Include any assumptions
• Include 3 or more (passing!) doctests
• Include 1-2 use cases (if necessary)

Function Implementation

• The solution is not is not copied from djeada
• The code is auto-formatted
• The code has no (reasonable) linting mistakes
• Variables are named with snake_case
• The function has a clear and helpful name
• The file's name matches the function name
• The code follows the strategy as simply as possible
• Variable names are clear and helpful
• Comments explain the strategy (if necessary)
• There are type annotations
• (challenge) The code includes defensive assertions

Unit Test Suite

• The test class has a helpful name in PascalCase
• The test class has a docstring
• Each unit test has
  • A helpful name
  • A clear docstring
  • Only one assertion
• All tests pass
• There are tests for defensive assertions
• There are tests for boundary cases
• Test suite includes black-box and glass-box tests