TEST: Rename converter/ to prompt_converter/ and add 19 converter tests

[romanlutz]

Summary

Renames the test directory to match the source package and adds unit tests for 19 previously untested converters.

Changes

Directory Rename

• \ ests/unit/converter/\ → \ ests/unit/prompt_converter/\ (matches \pyrit/prompt_converter/)

New Test Files (19)

All algorithmic converters — no LLM mocking needed:

Converter	Test Focus
AtbashConverter	Cipher encoding, digit reversal, append_description
Base64Converter	Multiple encoding functions (b64, urlsafe, b16)
CaesarConverter	Shift offset, wraparound, case preservation, validation
CharacterSpaceConverter	Space insertion, punctuation removal
FlipConverter	Text reversal, palindromes
MorseConverter	Morse encoding, word separators
ROT13Converter	ROT13 cipher, roundtrip verification
SearchReplaceConverter	Regex patterns, list replacements
StringJoinConverter	Character joining with custom separators
SuffixAppendConverter	Suffix appending, empty suffix validation
UnicodeReplacementConverter	\\uXXXX format, space encoding
UnicodeSubstitutionConverter	High Unicode codepoint mapping
UrlConverter	URL encoding of special characters
RandomCapitalLettersConverter	Percentage-based capitalization
EmojiConverter	Letter-to-emoji mapping
AsciiArtConverter	ASCII art font rendering
AsciiSmugglerConverter	Unicode Tag encode/decode roundtrip
SneakyBitsSmugglerConverter	Invisible bit-level encode/decode
VariationSelectorSmugglerConverter	Variation Selector encode/decode

Testing

All 116 new tests pass locally. Each test covers basic conversion, edge cases (empty strings), unsupported input types, and input/output type support.