fix: reject a timer_id over 255 characters before the INSERT

[lesnik512]

Rejects a timer_id over 255 characters with ValueError before the INSERT, the same way queue is already rejected.

Why

timer_id is a String(255) column, but nothing checked its length before the INSERT. What happened next depended on the SQLAlchemy version:

• SQLAlchemy 2.0.0: binds the value as $1::VARCHAR(255). Postgres truncates an explicit cast instead of raising, so the row landed with its timer_id cut to 255 characters. Two different timers sharing their first 255 characters then deduplicated against each other, and the second publish returned None. I confirmed the cast by compiling the INSERT with 2.0.0. The floors 3.11 and 3.12 jobs on refactor: move the outbox insert and its NOTIFY into the client #237 showed the truncation.
• Newer SQLAlchemy: the bind has no length, so Postgres raised and the caller got an opaque DBAPIError.

The change

_validate_publish_args now checks timer_id length next to queue. It runs for broker.publish, publisher.publish and OutboxResponse, which raises when it's constructed, like its other checks. The message matches the one the fake client already raised: timer_id must be at most 255 characters (got N).

Tests

Written first and failing:

• test_publish_command_rejects_oversize_timer_id and test_outbox_response_rejects_oversize_timer_id;
• test_publish_rejects_an_oversize_timer_id_without_storing_a_row, on real Postgres. It failed on current SQLAlchemy with a DBAPIError. On 2.0.0 it would have stored a truncated row; the floors jobs run it there.

test_publish_command_accepts_timer_id_at_the_column_limit pins the boundary.

Verification

• 741 tests pass in docker at 100% coverage.
• ruff and ty are clean.

[github-actions]

Benchmark gate

✅ gate passed

scenario	msg/s	delete/msg	WALrec/msg	WALB/msg	fpi	upd	del	dead_tup
consumer/w1/b10	1442	1.000	6.69	913	242	5000	5000	10000
consumer/w1/b100	1579	1.000	7.09	1143	243	5000	5000	10000
consumer/w2/b10	1795	1.000	6.70	915	242	5000	5000	10000
consumer/w2/b100	2344	1.000	7.09	1143	243	5000	5000	10000
consumer/w4/b10	1887	1.000	6.75	947	242	5000	5000	10000
consumer/w4/b100	2251	1.000	7.09	1142	243	5000	5000	10000
consumer/w1/b100/tfbs100	8915	0.010	6.06	1117	243	5000	5000	10000
consumer/w1/b100/interval1-10	1509	1.000	7.15	1186	265	5000	5000	10000
producer/w1/b100	5851	0.000	3.04	597	1	0	0	0
producer/w1/b100/ppt1	1231	0.000	4.04	637	1	0	0	0

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