Copy unresolved failed messages and their bodies to SQL

[warwickschroeder]

What this adds

The migration now copies unresolved and retry-issued failed messages, with their failure groups and bodies, from RavenDB to SQL Server or PostgreSQL while ServiceControl is closed. It does not copy archived or resolved messages yet, and a Release build still refuses to copy anything.

	Files	Lines
Code	31	+1,344 −198
Tests	31	+3,819 −34
Docs	3	+25 −17

• Reader: FailedMessagesReader reads only unresolved messages through a RavenDB index, and reads the whole collection if the index can't be trusted.
• Writer: FailedMessagesWriter writes one row per message, from its newest attempt, plus its failure groups.
• Bodies: MigrationBodyLoader reads them from RavenDB, and they are stored inline or externally by the same rule ingestion uses.
• New skip reasons: a bad id, no attempts, a value too long, a malformed value, a missing body.
• SQL Server: the insert is faster.
• Stalls: a stalled category no longer hangs startup.
• Debug switch: SERVICECONTROL_MIGRATION_ALLOW_UNRELEASED lets a Debug build copy for real for testing. This will be removed once the migration engine is ready for release.
• Docs: docs/migration/ is updated to match.

Tests

• ServiceControl.UnitTests/Migration: body loading, stalls, refusal wording and the Debug switch.
• ServiceControl.Persistence.Tests/EFCore/Migration: mapping, skips, groups and bodies.
• ServiceControl.Persistence.Tests.RavenDB/DataMigration: reading through the index and the fallbacks, resuming, and body attachments.
• ServiceControl.Persistence.Tests.SqlServer and .PostgreSql: value length checks and the SQL Server insert.
• ServiceControl.Migration.AcceptanceTests: an end-to-end copy of failed messages and their bodies.