fix(agent): empty multimodal content becomes None

[1688mengdie]

Problem

An empty multimodal message (no text and no images) is converted through
impl From<Message> for AIMessage into content: Some("") - a bare empty
string sent straight to the provider, where a strict API rejects it (400) and
a permissive gateway silently bills an empty round.

Root cause

The Multimodal conversion arm has no empty-content guard. It starts content
from the text value, only appends attached-image markers when images are
present, then always wraps the result in Some(content). The Text and Mixed
arms both guard empty content (the Mixed arm returns None for empty text),
so the Multimodal arm is the only one without defence.

Fix

After building content, return None when it trims to empty. Non-empty
multimodal content (text, images, or both) is unaffected, and an image-only
message keeps its image markers. The user_multimodal constructor is left
unchanged: it returns Self and cannot reject, and serde deserialization
rebuilds the fields directly, so the conversion arm is the correct chokepoint
to defend. No new helper or configuration is introduced.

Testing

• Test degree: tested (focused behavioral tests).
• Added 4 behavioral cases in message.rs: empty text+images becomes None;
  non-empty text preserved; image-only kept; constructor-fed conversion.
• cargo test -p bitfun-core --features agent-runtime --jobs 4 - message::tests
  green (9 passed); one unrelated pre-existing coordinator test still fails on
  the clean baseline.
• AI-assisted: yes (generated with review; commands above recorded).

Closes #2676

Commit list:

• d6f774179 fix(agent): empty multimodal content becomes None - message.rs
  Multimodal arm content -> None; adds 4 behavioral tests.