vertexai: fix handling of single string in add_texts

[ykoh42]

PR Description

Fix an issue where passing a single string to add_texts would split it into individual characters instead of treating it as a single document. This happens because Python strings are iterable, so list(texts) would convert a string like "hello" into ["h", "e", "l", "l", "o"]. Added a check to automatically wrap a single string in a list.

Relevant issues

Type

🐛 Bug Fix
✅ Test

Changes(optional)

• Added type checking in add_texts to detect single strings and wrap them in a list
• Added unit test to verify the fix works as expected

Testing(optional)

Added a new unit test test_add_texts_with_single_string that verifies a single string gets properly wrapped in a list before being processed.

Note(optional)

This bug would cause quiet failures where users passing a single string would get unexpected results - the string would be split into individual characters and each character would be embedded as a separate document. The fix ensures a more intuitive API behavior where strings are handled as single documents.

[lkuligin]

Since the signature is texts: Iterable[str],, I'm not sure a user should even pass a single string :)