Raise TypeError when string is passed as IndexDefinition prefix

[Br1an67]

Description of change

Passing a string to the prefix parameter of IndexDefinition causes it to be iterated character-by-character, resulting in each character being treated as a separate prefix. For example:

defn = IndexDefinition(prefix="MY:KEY:", index_type=IndexType.HASH)
# Actually indexes hashes prefixed by 'M', 'Y', ':', 'K', 'E', ...

This adds a type check in _append_prefix that raises TypeError with a helpful error message suggesting to wrap the string in a list:

TypeError: prefix must be a list of strings, not a single string. Did you mean prefix=['MY:KEY:']?

Pull Request check-list

• Do tests and lints pass with this change?
• Is the new or changed code fully tested?
• Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
• Is there an example added to the examples folder (if applicable)?

Fixes #3890

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Note

Low Risk
Low risk: adds a small input validation check and a targeted test; the only behavioral change is that previously-accepted (but buggy) string prefix values now raise TypeError.

Overview
Prevents accidental character-by-character prefix expansion by making IndexDefinition(prefix=...) raise TypeError when prefix is a single string (with a hint to wrap it in a list).

Adds a regression test ensuring IndexDefinition(prefix="...") fails with the expected error.

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[petyaslavova]

Hi @Br1an67, thank you for your contribution! We will have a look at it.