Check that log strings tests match on are still ones the daemon emits

A test that greps the daemon's log for a message the daemon stopped emitting
does not fail. It stops observing, and an assertion built on it — especially
one expecting a count of zero — then passes because nothing can be seen
rather than because nothing happened. Several findings have come from that
one class, so it is now checked mechanically.

testing/check-log-strings.py extracts the strings test code matches against
daemon log text and requires each to exist in src/. It reads four shapes:
python `"..." in line` tests, the first argument of the shell log helpers,
bash associative-array pattern tables, and greps whose input is daemon log
output. Patterns carry regex syntax, so each is reduced to the longest
literal run of every alternation branch, with escaping resolved in the same
pass — `\.` is a literal dot and `.` is a wildcard, and conflating them would
let a pattern match text that is not there.

Scoping is by what a grep READS, not by what its file mentions. A suite greps
its own analyzer output, fipsctl JSON, Tor's log and ping output, and none of
those have to correspond to a string in src/; scoping on the file produced 34
false positives. Strings that legitimately do not come from src/ are named in
ALLOWED with a reason, so the exceptions are reviewable rather than invisible.

It found six dead strings, three of which were not previously known: a second
copy of "Excessive decrypt failures" in the interop pattern table, a dead
"Handshake error" branch inside an alternation whose other branches still
matched, and "bootstrap failed" in the NAT suite. All six are corrected to
what the daemon emits, or dropped where the surviving branch already covers
the case. Verified by breaking it two ways and confirming it reports.

Also here, since it is the same failure shape one layer down: the shared log
library returned an empty string for a container whose logs it could not
read, so analysing a missing container reported no panics, no errors and no
sessions, and exited 0 — indistinguishable from a clean run. It now reports
which containers could not be read and raises, matching the fix already
landed in the chaos copy.

.github/workflows/ci.yml

@@ -61,6 +61,8 @@ jobs:
         run: pip3 install --quiet pyyaml
       - name: Check local and GitHub runners cover the same work
         run: bash testing/check-ci-parity.sh
+      - name: Check test log matchers against the strings src/ emits
+        run: python3 testing/check-log-strings.py
 
   fmt:
     name: Format check