fix: don't freeze Problem so it survives yield-dependency unwinding (#9)

[PythonFZ]

Fixes #9.

Problem

Raising a Problem through (or behind) a FastAPI yield dependency returned a generic 500 instead of the intended status:

@app.get("/via-yield")
async def via_yield(_=Depends(yield_dep)):
    raise Missing(detail="nope")   # -> 500, not 404

Root cause

A Problem is also an Exception. FastAPI runs yield dependencies inside an AsyncExitStack; as the exception unwinds back out, contextlib reassigns exc.__traceback__ at the Python level:

File ".../contextlib.py", line 255, in __aexit__
    exc.__traceback__ = traceback
File "<string>", line 4, in __setattr__
dataclasses.FrozenInstanceError: cannot assign to field '__traceback__'

Problem was a frozen=True Pydantic dataclass, so that assignment raised FrozenInstanceError, which masked the real problem and fell through to the catch-all 500 handler. Routes without yield dependencies never hit the Python-level reassignment (CPython sets __traceback__ at the C level on the initial raise), which is why it went unnoticed.

Fix

An exception must be writable at the Python level by design, so rather than special-casing __setattr__, the Problem dataclass is simply no longer frozen:

• frozen=False, eq=False in _ProblemMeta — eq=False keeps an exception's normal identity-based equality and hashing (a non-frozen dataclass would otherwise synthesize value-based, unhashable instances).
• Dropped the frozen_default=True @dataclass_transform hint so the static contract matches runtime (no "frozen" claim the runtime doesn't enforce).

The handlers already treat a raised problem as read-only (build_wire never mutates it), so reusing a shared module-level constant across requests stays safe — now verified by test_state_safety_shared_problem_across_paths rather than relying on immutability.

Tests (TDD — written first, watched fail with FrozenInstanceError/500, then fixed)

• test_yield_dependency.py
  • issue scenario: raise through a yield dependency → real status, not 500
  • raise through nested yield deps → detail, instance, and typed extension fields all survive (no information lost)
  • catch one problem and escalate to a different problem type through a guarded route → response reflects the new type/status
• test_problem.py
  • test_fields_are_mutable_problem_is_not_frozen (replaces the old frozen-mutation test)
  • test_exception_dunders_reassignable_with_no_info_lost — __traceback__ / __cause__ / __context__ / __suppress_context__ / add_note all reassignable, values intact

Full suite: 92 passed. ruff check, ruff format, and pyright all pass.

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