fix(ops): fail fast on missing backup method

[leon-ape]

Fixes #10816.

Problem

A Backup may report status.phase: Completed while status.backupMethod is empty. The restore plan detects that invalid state before any Restore exists, but its non-terminal error currently leaves the horizontal-scaling OpsRequest retrying instead of failing.

Changes

• keep detection of the incomplete Backup status in the shared restore plan
• classify the plan's RestoreFailed result as fatal at the scale-out Operations boundary
• verify the completed-without-backupMethod case is terminal and creates no Restore

Scope

This PR only changes missing-backupMethod error classification for scale-out restore. It does not change restore environment propagation, namespaces, authorization, VolumeSnapshot behavior, or source-target selection.

This is the narrow fail-fast replacement for the corresponding part of #10597.

Validation

• scripts/codex-go-test.sh ./pkg/operations -count=1
• git diff --check

[apecloud-bot]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.69%. Comparing base (98453e1) to head (3032839).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10818      +/-   ##
==========================================
+ Coverage   65.61%   65.69%   +0.07%     
==========================================
  Files         510      510              
  Lines       64403    64405       +2     
==========================================
+ Hits        42259    42309      +50     
+ Misses      18382    18346      -36     
+ Partials     3762     3750      -12     

Flag	Coverage Δ
unittests	65.69% <100.00%> (+0.07%)	⬆️

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[leon-ape]

/approve