logging: Add regression coverage for rotated file mode

[steadytao]

Adds Caddy-side regression coverage for the log rotation mode issue addressed by the timberjack bump in #7618.

Why

Issue #7613 reported that after log rotation, the new active log file could lose permissions configured via mode because the process umask still masked bits during rotation-time file creation.

The behavioral fix belongs in github.com/DeRuina/timberjack and was brought into Caddy by #7618, which bumps timberjack to v1.4.1.

This PR does not change Caddy runtime behavior. It adds regression coverage to ensure Caddy’s integration continues to preserve the configured file mode across rotation under a nonzero umask.

What this tests

The new test exercises the user-facing behavior from #7613:

• umask set to 0022
• rolling file logs enabled
• configured file mode 0660
• rotation triggered
• asserts that the new active log file still has mode 0660

Related

• Regression coverage for Log file permissions not preserved after rotation (umask applied instead of configured mode) #7613
• Relies on the timberjack fix brought in by chore: bump timberjack to v1.4.1 #7618
• Upstream timberjack issue: [BUG] Configured FileMode is not preserved after rotation when umask masks bits DeRuina/timberjack#90

Assistance Disclosure

No AI was used.

[francislavoie]

LGTM, needs a rebase to fix go.mod then we can merge.