FEAT: Add JsonStringConverter for JSON-safe string conversion (#454)

[akashWhoCodes]

Description

Adds a new JsonStringConverter that uses json.dumps() to properly escape strings for JSON embedding, addressing issue #454.

Changes:

• Add JsonStringConverter class that escapes special characters for JSON embedding
• Add comprehensive unit tests with pytest fixture following PyRIT best practices
• Update __init__.py to export JsonStringConverter
• Update HTTP target example to demonstrate JSON string converter usage
• All tests pass and follow PyRIT style guide

Use case: When sending prompts to HTTP targets expecting JSON payloads, strings need proper escaping. Previously this required SearchReplaceConverter(pattern=r"(?! )\s", replace="") which only stripped whitespace. The new converter handles all JSON escaping correctly (quotes, newlines, tabs, backslashes, unicode).

Example:

from pyrit.prompt_converter import JsonStringConverter

converter = JsonStringConverter()
result = await converter.convert_async(prompt='Hello "World"\nNew line', input_type="text")
# result.output_text == 'Hello \"World\"\\nNew line'

Closes #454

Tests and Documentation

• Unit tests: 12 tests in tests/unit/converter/test_json_string_converter.py covering:
  • Basic conversion
  • Special character escaping (quotes, newlines, carriage returns, tabs, backslashes)
  • Complex strings with multiple escape sequences
  • Unicode characters
  • Empty strings
  • Input/output type validation
• All tests pass: uv run pytest tests/unit/converter/test_json_string_converter.py -v
• Documentation: Updated doc/code/targets/10_http_target.py and 10_http_target.ipynb to use JsonStringConverter instead of SearchReplaceConverter

[akashWhoCodes]

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[akashWhoCodes]

Pushed updates to address the earlier CI failures.
Happy to address any remaining failures once the checks run. Thanks!

cc: @romanlutz