fix(security): enforce guild allowlist for DMs

[minto-dane]

Motivation

• The runtime allowed guild_id == None messages unconditionally, which lets Discord direct messages bypass a configured guild allowlist while the bot subscribes to GatewayIntents::DIRECT_MESSAGES.
• The change makes OO_ALLOW_GUILD_IDS behave as an actual allowlist boundary so DMs are not implicitly considered allowed when an allowlist is in effect.

Description

• Change TrustedCore::allow_guild to treat guild_id == None as allowed only when runtime.allow_guild_ids is empty by returning self.runtime.allow_guild_ids.is_empty() for the None case.
• Preserve existing deny-list semantics so deny_guild_ids still blocks listed guilds and an empty allow_guild_ids keeps previous permissive behavior.
• Add a regression test allowlist_blocks_direct_messages_without_guild that verifies a message with guild_id: None is suppressed with SuppressReason::GuildDenied when allow_guild_ids is non-empty.

Testing

• Ran cargo fmt, which completed successfully.
• Attempted cargo test allowlist_blocks_direct_messages_without_guild, but the run was blocked when the lindera-ipadic build script failed to download dictionary assets and the test suite could not complete.
• Attempted cargo test --no-default-features allowlist_blocks_direct_messages_without_guild to avoid the asset download, but it was also blocked by the same lindera-ipadic asset download failure.

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Codex Task