refactor: extract build_sender_context pure function + unit tests

[chaodu-agent]

Context

Follow-up from PR #583 review. The SenderContext assembly logic is currently embedded inside EventHandler::message() in src/discord.rs, making it difficult to unit test without mocking serenity.

Proposal

Extract a pure function build_sender_context() that takes the resolved values (in_thread, parent_id, msg_channel_id, author info, etc.) and returns a SenderContext. This enables table-driven tests covering:

• in-thread message → channel_id = parent, thread_id = Some(thread)
• non-thread message → channel_id = channel, thread_id = None
• error/non-guild fallback → channel_id = channel, thread_id = None

Similar to how detect_thread() was already extracted as a pure function.

References

• PR fix(discord): use parent channel_id in SenderContext when in thread #583
• Issue agent get thread id, instead of channel id #581

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