Tighten mypy and ruff configuration

[sandhose]

Summary

• Replace deprecated logger.warn() with logger.warning()
• Add missing type annotations and tighten mypy config (disallow_untyped_defs now passes on all modules)
• Narrow type: ignore comments to specific error codes and enable warn_unused_ignores
• Enable warn_return_any in mypy and fix all violations
• Enable ruff rules: UP (pyupgrade), B (bugbear), C4, SIM, RUF, PIE, RSE, G, PERF, TC
• Drop explicit target-version from ruff config (inferred from requires-python)
• Modernize type annotations to Python 3.10+ syntax (dict not Dict, X | Y not Union)
• Add from err to raise-in-except blocks, merge nested with statements, annotate ClassVar
• Remove dead ProxyConnectError exception class (unused after Twisted removal)
• Remove # -*- coding: utf-8 -*- declarations (unnecessary since Python 3)

Notes

• _choppable_get in apnstruncate.py now raises ValueError instead of implicitly returning None for unknown choppable types. The branches are exhaustive so this is unreachable in practice.
• warn_return_any = False for sygnal.apnstruncate module — needed due to dict[str, Any] access patterns.
• PERF203 (try-except in loop) is suppressed — the loops have intentional error handling.

Test plan

• uv run mypy sygnal/ tests/ passes with zero errors
• uv run ruff check sygnal/ tests/ passes
• uv run pytest tests/ — 55 tests pass
• CI green

Part 6 of 7 in the repository modernisation series. Builds on #456.