fix: link the 0.15 upgrade guide from the pending-index drift error

[lesnik512]

Follow-up to #229 for 0.15.0, no issue.

Every table migrated before 0.15 fails validate_schema() until _pending_idx is rebuilt. The error was accurate but gave no hint about what to do:

missing index over ['queue', 'next_attempt_at', 'id'] (name='outbox_pending_idx', unique=False)

When the missing index is _pending_idx, the line now ends with a link to the "Upgrading to 0.15" section of the Alembic guide:

...; a table created before 0.15 needs this index rebuilt: https://faststream-outbox.modern-python.org/operations/alembic/#upgrading-to-015

The base URL is the one _AUTOGEN_BLIND_HINT already uses, and I checked that the built docs render the upgrading-to-015 anchor. A table that's missing the index entirely gets the same pointer, which still helps: that section shows the index's exact shape.

Test: the existing test_validate_schema_reports_the_pre_0_15_pending_index_shape now also requires the link, and it failed before the change.

Verification: 692 tests pass in docker at 100% coverage, ruff is clean, ty shows the same 6 diagnostics as main, and the docs build.

[github-actions]

Benchmark gate

✅ gate passed

scenario	msg/s	delete/msg	WALrec/msg	WALB/msg	fpi	upd	del	dead_tup
consumer/w1/b10	1297	1.000	6.70	914	242	5000	5000	10000
consumer/w1/b100	1372	1.000	7.09	1143	243	5000	5000	10000
consumer/w2/b10	1590	1.000	6.70	915	242	5000	5000	10000
consumer/w2/b100	1842	1.000	7.09	1143	243	5000	5000	10000
consumer/w4/b10	1626	1.000	6.72	943	242	5000	5000	10000
consumer/w4/b100	1891	1.000	7.13	1178	263	5000	5000	10000
consumer/w1/b100/tfbs100	6279	0.010	6.07	1119	244	5000	5000	10000
consumer/w1/b100/interval1-10	1409	1.000	7.09	1143	243	5000	5000	10000
producer/w1/b100	3038	0.000	3.04	597	1	0	0	0
producer/w1/b100/ppt1	748	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).