feat(proxy): support configurable timeouts and normalize Anthropic system messages

[matrix72c]

Summary

• allow SessionClient upstream timeouts to be configured with aiohttp.ClientTimeout or a serializable dict while preserving aiohttp defaults
• optionally move system role messages into the Anthropic top-level system field before forwarding and trajectory recording
• tolerate Anthropic batch payloads that do not have top-level messages

Both options are backward-compatible and disabled/defaulted to the existing behavior unless explicitly configured.

Motivation

External coding agents may run longer than aiohttp's default timeout. Some Anthropic-compatible backends also reject system roles inside messages, even when clients emit that shape. These proxy-level options let callers handle both cases without changing agent or daemon protocols.

Tests

• Black 24.10.0
• Flake8 7.1.1
• isort 5.13.2
• codespell 2.3.0
• pytest -q tests/test_adapters/test_session_client.py -k "client_timeout or anthropic_system or batch_request" (7 passed)