Fix MCP headers config and hot update middleware

[melonelish]

Summary

• make _add_or_update_server(..., headers=None) backward compatible
• preserve existing HTTP MCP headers when updating server config
• load HTTP MCP headers from FINCHBOT_MCP__{SERVER}__HEADERS
• fix nested MCP env parsing for FINCHBOT_MCP__{SERVER}__ENV__{KEY}
• fix MCP hot update middleware so sync wrappers do not treat asyncio.Task as a tool list
• add regression tests for MCP headers, env loading, and hot update middleware behavior

Tests

• uv run pytest -q