fix: int_to_roman silently returns wrong values for out-of-range input

[faizzyhon]

Problem

int_to_roman performed no input validation. Python's floor-division semantics silently corrupted results for out-of-range inputs:

# Before this fix:
int_to_roman(0)    # ''          <- silent wrong output
int_to_roman(-1)   # 'CMXCIX'   <- silently returns 999 (WRONG)
int_to_roman(-5)   # 'CMXCV'    <- silently returns 995 (WRONG)
int_to_roman(4000) # 'MMMM'     <- not a valid standard Roman numeral
int_to_roman(True) # 'I'        <- bool silently accepted as 1

Root cause: divmod(-1, 1000) returns (-1, 999) in Python, so 'M' * -1 yields '' and the remainder 999 propagates through the ROMAN lookup table, producing a plausible-looking but completely wrong answer.

Fix

Added early validation guards:

1. TypeError for non-integer types (including bool, which is a subclass of int)
2. ValueError for integers outside the valid range [1, 3999]
3. Added four doctest error cases so the guards are exercised by the doctest runner

Describe your change:

• Fix a bug or typo in an existing algorithm?

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.
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