test(agent): make the ctx-cancel matrix case deterministic

CI's test-core failed on
TestGenerateText_Matrix/geminicli/CanceledCarriesSentinelAndEvidence with
Stderr = "" and StdoutBytes = 0. That is the signature of the child never
having run: the test cancelled on a fixed `time.Sleep(50ms)`, which races
subprocess startup, so on a loaded runner the kill landed before sh executed
either printf. My test, my flake — a fixed delay is exactly the wrong tool for
synchronising with a subprocess.

The child now touches a sentinel file after writing both streams, and the
parent waits for that file before cancelling, so the cancel is ordered after
the writes by construction rather than by hope. The sentinel is per-subtest
because these run in parallel across five providers; a shared path would let
one provider's child release another's cancel. A 10s ceiling still cancels
regardless, so a genuinely stuck child fails loudly instead of hanging.

Checked that the fix did not simply defang the assertion: mutating
HandleTextGenResult's ctx branch back to a bare sentinel still fails this case
with "evidence lost". Stressed at -count=50 -cpu=1,2,4 (150 executions), clean.

Also split the table-driver into per-scenario functions. It had grown past
maintidx (complexity 25, MI 18) once the composition assertions and the two
stdout cases landed. Extracting is the right fix rather than a nolint, and
t.Parallel now lives at the single t.Run site so tparallel can see it. All 35
subtests (5 providers x 7 scenarios) preserved — verified by count, not by
assumption.

Verified: 8573 unit tests, lint 0 issues on a cleaned cache.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: peyton-alt