TST: Added test that checks if df.sum() results in concatenation instead of dtype coercion to float64 or int64

[AbelJSanchez]

Added test_sum_string_dtype_coercion() that checks if the df.sum() method results in in concatenation for numeric strings, and not coercion to dtype int64 or float64.

I wrote three different assertions:

1. Sum of two DataFrames with "integer" strings
2. Sum of two DataFrames with "floating point" string
3. Sum of two DataFrames with both number type strings.

• closes BUG: df.sum() of string columns depends on whether or not they can be coerced to numbers #22642
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.
• If I used AI to develop this pull request, I prompted it to follow AGENTS.md.