test(cline): pin the strip against the real focus-chain block

The `# task_progress RECOMMENDED` boundary change is only exercised by a
synthetic string, so nothing in the suite says what it does to the block
Cline actually sends. That block is pushed as its own content part and no
line inside it opens with `#` or `<`, so both searches miss and the whole
thing runs off the end — ending at the first boundary changes nothing for a
real transcript. Assert that with the text verbatim from cline's
`focus-chain/prompts.ts`, and add the non-ASCII case the byte-offset
splicing needs to survive.

Also assert the shrink invariant the environment loop now relies on. The
regression it replaces grew the string exponentially, so the test written
for it hangs the run and exhausts memory rather than failing; checked in
debug builds it fails on the first pass instead.