Fix ARM64 segfault on connect failure with libpq 506

[Dark-Athena]

Summary

• avoid broad failed-PGconn attribute introspection in finish_pgconn()
• build a safe failed-connection snapshot from explicit fields
• sanitize _connect() failure branches to pass conninfo-derived diagnostics
• add a regression test ensuring finish_pgconn() doesn't read arbitrary PGconn attributes
• document the fix in release notes

Validation

• reproduced on ARM64 remote env:
  • libpq 505.2: expected connection error (no crash)
  • libpq 506.0: segfault before patch, normal OperationalError after patch
• checked failed-connection diagnostics still expose pgconn.db and sane status values

Closes #30