fix: strip double quotes from sftp.args flags to prevent restic parse error

[wahajahmed010]

Fixes #1227

Problem

When configuring an SFTP repository with custom sftp.args containing quoted paths, Backrest 1.13.0 generates an invalid --option=sftp.args=... flag that restic cannot parse:

invalid argument "sftp.args=..." for "-o, --option" flag: parse error on line 1, column 30: bare " in non-quoted-field

This prevents users from modifying or saving SFTP repository configurations.

Fix

Updated sanitizeRepoFlags to strip double quotes from sftp.args values. Double quotes are not needed in the sftp.args context because restic passes these values directly to the sftp command. The -i @ workaround is preserved.

Added TestSanitizeRepoFlags with coverage for quote stripping, the -i @ workaround, and edge cases.

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