fix: allow # in channel names

[FearLessSaad]

Description

The send_to_user method is currently broken out of the box. It constructs a channel name using the # character, but the internal channel name validation regex explicitly rejects the # character, resulting in a validation error before the request is even sent.

Updated the CHANNEL_NAME_PATTERN regex in src/channel.rs to include the # character:

static CHANNEL_NAME_PATTERN: LazyLock<Regex> =
    LazyLock::new(|| Regex::new(r"^[#A-Za-z0-9_\-=@,.;]+$").unwrap());