Add failover storage settings to SQL Server allowed config

[Copilot]

SQL Server operations were emitting warnings for WALG_FAILOVER_STORAGES_CACHE_LIFETIME and WALG_FAILOVER_STORAGES_CHECK_TIMEOUT despite these being valid failover storage settings already supported by PostgreSQL and Greenplum.

Changes

• Added 11 failover storage configuration settings to SQLServerAllowedSettings in internal/config/config.go
• Added test to verify settings are recognized for SQL Server configuration

The settings include:

• WALG_FAILOVER_STORAGES
• WALG_FAILOVER_STORAGES_CHECK
• WALG_FAILOVER_STORAGES_CHECK_TIMEOUT
• WALG_FAILOVER_STORAGES_CACHE_LIFETIME
• WALG_FAILOVER_STORAGES_CACHE_EMA_* (6 EMA parameters)
• WALG_FAILOVER_STORAGES_CHECK_SIZE

This brings SQL Server to feature parity with other databases for failover storage configuration.

Original prompt

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This section details on the original issue you should resolve

<issue_title>[FEATURE] cleanup sql server warning</issue_title>
<issue_description>### What feature would you like?

when backup sql server, I see the following warnings. we need to remove them

C:\i\wal-bin>.\wal-g-sql.exe --config .\wal-g-sql.yaml delete retain FULL 3 --confirm
WARNING: 2026/02/15 12:17:21.587009 WALG_FAILOVER_STORAGES_CACHE_LIFETIME is unknown
WARNING: 2026/02/15 12:17:21.587639 WALG_FAILOVER_STORAGES_CHECK_TIMEOUT is unknown
WARNING: 2026/02/15 12:17:21.587639 We found that some variables in your config file detected as 'Unknown'.
  If this is not right, please create issue https://github.com/wal-g/wal-g/issues/new
INFO: 2026/02/15 12:17:21.589946 No backup found for deletion
PS C:\i\wal-bin>

Why is this feature useful?

No response</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [FEATURE] cleanup sql server warning #40

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