fix(agent): stop fabricating empty message content

[1688mengdie]

Problem

The Text arm of impl From<Message> for AIMessage injected a synthetic placeholder
for empty text to avoid sending empty content to the provider: empty user text became
"(empty message)", empty system text became "You are a helpful assistant.", and
any other empty text became a single space. The model then treated these fabricated
strings as a real user utterance or a system directive.

Root cause

The "avoid empty content" defensive intent was implemented by injecting a non-empty
synthetic token, which defeats the downstream converters' existing empty-content
handling (openai/gemini filter out empty content, anthropic skips an empty system
message) because a non-empty fabricated string bypasses those filters. The Mixed arm
already returns None for empty text and is the correct in-file precedent to reuse;
the Text arm did not.

Fix

Return None for empty text in the Text arm instead of fabricating a placeholder,
keeping the existing warn! diagnostic. Nothing synthetic reaches the provider, the
converters' empty-content semantics apply, and the normal non-empty path is unchanged.
No new helper or configuration is introduced.

Testing

• Test degree: tested (focused behavioral tests).
• Added 4 behavioral cases in message.rs: empty user/system/assistant text becomes
  None; non-empty text is preserved.
• 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 #2675

Commit list:

• cc7e90846 fix(agent): stop fabricating empty message content - message.rs Text arm
  placeholder -> None; adds 4 behavioral tests.