refactor: tighten runtime lifecycle and remove maintenance debt

[seratch]

This pull request improves reliability and maintainability across Realtime sessions, model retries, sandbox providers, logging, and lint configuration without changing public APIs.

Changes

• Removes unused private helpers and stale lint suppressions.
• Converts eager logging to lazy formatting.
• Marks shared mutable class state with ClassVar.
• Uses time.monotonic() for Realtime audio playback timing.
• Cancels and awaits Realtime guardrail and tool tasks during session close.
• Avoids creating untracked tasks from Realtime completion callbacks.
• Tracks Daytona PTY workers and Docker cleanup tasks through provider shutdown.
• Tracks Unix PTY file descriptor close tasks without allowing a blocked macOS os.close() to hang session teardown.
• Consolidates PTY output collection across Unix local, Docker, Blaxel, and Daytona.
• Consolidates E2B and Runloop rclone bootstrap and mount option handling.
• Consolidates retry error-chain and metadata normalization shared by provider advice and runner retries.
• Enables DTZ005, G004, RUF006, RUF012, and RUF100 after clearing the corresponding source debt.
• Adds regression coverage for Realtime timing and cleanup, sandbox lifecycle handling, bounded Unix PTY cleanup, rclone setup, and retry normalization parity.

Compatibility

• Does not change public APIs, import paths, dataclass field order, or __all__ membership.
• Preserves retry decisions, PTY output ordering, mount behavior, log contents, and exception information.
• Treats v0.17.7 as the compatibility boundary.

Verification

• make format, make lint, make typecheck, and make tests all pass.
• Focused Unix sandbox tests pass: 9 tests.
• All 21 unique auto-runnable sandbox entrypoints were validated successfully.
• After the Unix PTY cleanup fix, the affected UnixLocal PTY, UnixLocal runner, and local Temporal examples were rerun and all passed.
• External-provider entrypoints passed for Blaxel, Cloudflare, Daytona, E2B, Modal, and Runloop.
• Vercel and other auto-skipped helper/server entrypoints were not forced.