Complete MAVLink dataclass migration, receiver idiom cleanup, and Python 3.12 alignment

[mcroomp]

Summary

• finish migrating MAVLink send/receive callsites in ground station, simulation, calibrate tooling, and tests to dataclass-driven message APIs
• standardize receiver loops around decode_message(...) and typed branching (match/isinstance) where appropriate
• add/expand MAVLink dataclass coverage and decode registry in groundstation/gcs.py
• align repository Python baseline to 3.12 in project metadata, CI, and agent guidance
• clean remaining Python/Pylance warnings in touched surfaces

Key Changes

• groundstation/gcs.py
  • expanded message dataclasses and centralized decode/send helpers
  • refined connection typing with Protocols instead of broad Any
• simulation/rawes_lua_harness.py
  • added generic send_message(...) support for dataclass messages
• groundstation/rawes_modes.py
  • updated anchor send path to prefer send_message(...)
• migrated additional send/receive callsites in:
  • calibrate/*
  • simulation/mediator.py
  • scripts/query_hardware.py
  • tests/unit/*, tests/simtests/*, tests/sitl/*, tests/hil/*
• baseline/config updates:
  • pyproject.toml (Python >= 3.12)
  • .github/workflows/ci.yml (3.12)
  • AGENTS.md guidance for modern Python and 3.12 floor

Validation

• unit tests: 467 passed
• simtests: 14 passed, 5 skipped
• SITL stack (bash test.sh stack -n 4): 7 passed, 0 failed

Notes

• remaining diagnostics are limited to Lua analyzer warnings in scripts/rawes_test_surface.lua (outside Python warning cleanup scope).