REFACTOR: Promote exception status codes & messages to ClassVar defaults

[Roman Lutz (romanlutz)]

Summary

Promote hard-coded status_code / message defaults out of the
__init__ signatures of PyritException and its subclasses in
pyrit/exceptions/exception_classes.py and onto each class as
DEFAULT_STATUS_CODE: ClassVar[int] / DEFAULT_MESSAGE: ClassVar[str],
in line with the class-level constants rule in the style guide and the
pattern established by #1964 and #1965.

Changes

For each of the following exception classes, the hard-coded
status_code / message defaults in the __init__ signature are
replaced with per-class DEFAULT_* ClassVar constants resolved
through the sentinel-default pattern (None → self.DEFAULT_*):

Class	DEFAULT_STATUS_CODE	DEFAULT_MESSAGE
PyritException	500	"An error occurred"
BadRequestException	400	"Bad Request"
RateLimitException	429	"Rate Limit Exception"
ServerErrorException	500	"Server Error"
EmptyResponseException	204	"No Content"
InvalidJsonException	—	"Invalid JSON Response"
MissingPromptPlaceholderException	—	"No prompt placeholder"

InvalidJsonException and MissingPromptPlaceholderException only
parameterize message, so only DEFAULT_MESSAGE is defined for
them — they inherit DEFAULT_STATUS_CODE (500) from PyritException.

The keyword-only signature (*,) is preserved for every class. All
existing call sites already pass these arguments by name (verified by
grep across pyrit/, tests/, doc/), so this is signature-compatible.

Verification

uv run --link-mode=copy ruff check pyrit/exceptions/    # All checks passed!
uv run --link-mode=copy pytest tests/unit/exceptions/   # 84 passed

Spot-checked dependent tests that import these exceptions:
tests/unit/score/test_insecure_code_scorer.py,
tests/unit/prompt_converter/test_generic_llm_converter.py,
tests/unit/executor/attack/multi_turn/test_tree_of_attacks.py,
tests/unit/prompt_normalizer/test_prompt_normalizer.py — 146 passed.

Notes

• No value changes. No behaviour changes. Pure structural refactor.
• Follows the pattern established in REFACTOR: Promote single-class module constants to ClassVar (backend + registry + setup) #1964 (backend/registry/setup) and
  REFACTOR: Promote single-class module constants to ClassVar (prompt_converter) #1965 (prompt_converter).

[Roman Lutz (romanlutz)]

Closing per maintainer feedback on the constants-audit work.

This PR introduced the sentinel-default pattern (changing x: T = <literal> into x: T | None = None + if x is None: x = self.DEFAULT_X) which widens the public API signature. The preference is to keep constants inside their owning class (Track A — see #1964, #1965, #1972, #1973, #1974, #1975) but not to promote hard-coded single-use defaults via a None-sentinel.

Branch left intact in case any portion is worth cherry-picking later.