Demangle Cython __pyx_* symbols in native stack frames

Native crash frames generated by Cython-compiled code appear as mangled C
symbols (e.g. __pyx_pw_6module_3func) in sentry-native stack traces, making
them unreadable without manual demangling. The Python SDK captures Cython
frames correctly at the Python level (.pyx file, line number), but when a
crash occurs in the native layer of a Cython extension, the native stack frames
shown in Sentry are opaque.

Current behavior

• Python-level Cython frames (captured by sentry-python via sys.exc_info())
  show correct .pyx source file and line number
• Native frames from the C code generated by Cython (captured by sentry-native
  on crash) show raw mangled symbols: __pyx_pw_6module_3func_1name,
  __pyx_f_6module_3ClassName_method, etc.
• C++ demangling (__cxa_demangle) does not apply to Cython's naming scheme
• No Cython-specific demangling exists in sentry-python or the Sentry backend

Gap

Cython uses a deterministic, documented symbol naming convention for the C code
it generates. A demangler can reconstruct the original Python-facing name from
a __pyx_* symbol:

• __pyx_pw_<module>_<n><name> → Python wrapper for <module>.<name>
• __pyx_f_<module>_<ClassName>_<method> → C-level function for
  <module>.<ClassName>.<method>

This demangling could be applied as a post-processing step in the
GnuBacktraceIntegration or as a standalone CythonIntegration, enriching
native frames with human-readable Cython source names before the event is sent
to Sentry.

Options

1. Extend GnuBacktraceIntegration to detect and demangle __pyx_* symbols
   in parsed backtrace frames — lowest friction, reuses existing pipeline
2. Add a standalone CythonIntegration event processor that post-processes
   all native frames across exception and crash events — cleaner separation,
   applicable beyond GnuBacktrace-formatted strings
3. Implement server-side demangling in the Sentry symbolicator for __pyx_*
   symbols — transparent to SDK users, but requires changes outside this repo

References

• Cython generated C naming conventions: https://cython.readthedocs.io/en/latest/src/userguide/debugging.html
• Existing GnuBacktraceIntegration: sentry_sdk/integrations/gnu_backtrace.py

Action taken on behalf of Angelo de Voer.