Standardise FlagNotFound error

[Zaimwa9]

Problem
Currently, when a flag key isn't found, the SDK returns a generic FlagsmithAPIError with the message API returned invalid response.
This forces consumers (cf OpenFeature crate) to rely on fragile string matching to detect this case.

Proposed solution
Add a FlagNotFound variant to ErrorKind and return it from Flags::get_flag when the key doesn't exist in the HashMap.

Requirements

• Add FlagNotFound error kind and return it from Flags::get_flag
• Update the OpenFeature provider to use the new error kind instead of string matching