snapshot: ignore stale lock delete failures

[ousamabenyounes]

Summary

• keep snapshot creation going when deleting an already-stale repository lock fails
• leave stale lock cleanup as an opportunistic step so stores with read/write but no delete permission can still run backups
• add a regression test using the mock store to deny lock deletion after a stale lock is present

Issue

Fix PlakarKorp/plakar#1670

Test verification (RED -> GREEN)

RED on unmodified origin/main with only the new regression test/helper applied:

--- FAIL: TestCreateIgnoresStaleLockDeleteFailure (0.02s)
    builder_test.go:71:
        Error:       Received unexpected error:
                     mock lock delete denied
FAIL
FAIL    github.com/PlakarKorp/kloset/snapshot    0.036s

GREEN on the patched branch:

ok      github.com/PlakarKorp/kloset/snapshot    0.035s

Full local validation

Baseline origin/main via local run-ci.sh: pass.
Final branch via the same run-ci.sh: pass.

run-ci.sh runs:

go build -v ./...
go test -v -coverprofile=coverage.out -covermode=atomic -timeout 1m ./...
go vet ./...

Diff coverage gate:

diff-coverage-gate: PASS no changed production lines with coverage data

[mathieu-plak]

Yeah I'm unsure about that one. That's just one part of the story. Now maintenance is blocked forever from running. The ticket suggests that maintenance should do the cleanup but if you don't have write... you don't have write access.