Validate app log sources before opening output files

[fallintoplace]

Changes

• validate --source before app lookup, token acquisition, and output-file creation
• preserve existing output files when source validation fails
• add command-level regression coverage

Why

apps logs opened --output-file with truncation before validating --source. A typo in the source filter could therefore erase an existing file even though the command immediately returned an argument error.

Tests

• go test ./cmd/apps ./libs/apps/logstream

[github-actions]

Waiting for approval

Based on git history, these people are best suited to review:

• @jamesbroadhead -- recent work in cmd/apps/

Eligible reviewers: @MarioCadenas, @Shridhad, @atilafassina, @calvarjorge, @ditadi, @fjakobs, @igrekun, @keugenek, @pffigueiredo, @pkosiec

_{Suggestions based on git history. See OWNERS for ownership rules.}

[github-actions]

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• PR number: 6133
• Commit SHA: 11175e5e8e41e40db4b2d2bf21cf098db481c1ae

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