fix(poll-loop): don't mistake </message> in body text for the end-of-message tag

[markbala]

Type of Change

• Fix - bug fix or security fix to source code

Description

The bug

If an agent's reply contains </message> inside its body — e.g. in a
code example or while explaining the destination format to a user —
the parser mistakes it for the closing tag of the wrapping
<message to="..."> block and silently drops everything after.

Example

User asks Andy:

How does the destination wrapping work?

Andy replies (full text):

<message to="mark">
Every reply lives inside a `<message to="name">…</message>` block.
The "to" name picks which chat receives it.
</message>

Before the fix: the user receives only

Every reply lives inside a `<message to="name">…

— the parser hit the </message> inside the inline backticks and stopped
there, dropping the rest of the explanation.

After the fix: the user receives the full reply.

The fix

Replace the non-greedy ([\s\S]*?) regex in dispatchResultText with a
scanner that finds the opening tag, then walks forward looking for
</message> while skipping inline backtick spans and ``` fenced blocks.
Additive — same behavior for any input that doesn't contain </message>
inside code.

Verification

• bun test container/agent-runner/src/poll-loop.test.ts — 6 new tests
  for findClosingMessageTag covering plain close, inline-backtick skip,
  fenced skip, outer-close-despite-inner-fenced-close, unclosed → -1,
  lone-backtick → -1. All 34 tests pass.