chore: give every coverage pragma a reason (TS6)

[lesnik512]

Brings the repo in line with TS5 and TS6: every # pragma: no cover now carries its reason on the same line. exclude_also was already ["if typing.TYPE_CHECKING:"] and no package file is omitted.

Of the 23 bare pragmas:

• Tested: 9. The upstream-mirrored fall-throughs in OutboxSubscriber.consume / process_message are reachable from user code. New tests in tests/test_fake.py cover them: a custom parser= that raises, a handler filter= that matches nothing, a middleware whose after_processed suppresses the no-handler error, and a handler raising StopConsume or SystemExit (with a FastStream app in context). The "unreachable" comments on those lines were wrong, so they are removed.
• Deleted: 0.
• Reasoned: 14. These are the optional asyncpg import fallback, the TestBroker abstract create_publisher_fake_subscriber, the Postgres-unreachable skip in two fixtures, the timeout/failure tails of test polling helpers, a drain-guard stub, and the lease-steal block in test_batched_flush_lease_lost_row_redelivers. I checked that last one: with the pragma removed, coverage on Python 3.11 sometimes misses steal_conn.execute, so it stays. The three-line comment above the row not deleted poll is folded into its same-line reasons.

Local checks:

• just install: ok
• just lint-ci: pass
• just test-ci against Postgres 17 (compose settings): 629 passed, coverage 100% on Python 3.14. It also passes at 100% on Python 3.11.

[github-actions]

Benchmark gate

✅ gate passed

scenario	msg/s	delete/msg	WALrec/msg	WALB/msg	fpi	upd	del	dead_tup
consumer/w1/b10	994	1.000	6.77	911	242	5000	5000	10000
consumer/w1/b100	1020	1.000	6.83	920	243	5000	5000	10000
consumer/w2/b10	1139	1.000	6.81	973	242	5000	5000	10000
consumer/w2/b100	1334	1.000	6.72	953	243	5000	5000	10000
consumer/w4/b10	1036	1.000	6.82	996	261	5000	5000	10000
consumer/w4/b100	1559	1.000	6.86	1037	244	5000	5000	10000
consumer/w1/b100/tfbs100	5295	0.010	6.07	1116	243	5000	5000	10000
producer/w1/b100	2118	0.000	3.04	584	0	0	0	0

Gated (fails the build): delete_calls + tuple counters (upd/del/ins) + the producer's insert_calls, exact; select_calls within +2; wal_records within a 10% band. msg/s, WAL bytes and total calls are informational (timing/FPI noise).