feat(testing): add test methodology and report tooling

[kvinwang]

Summary

• document the dstack test methodology, test-case authoring rules, and report output contract
• add dstack-test commands with consistent subcommand naming for running cases/plans, finalizing, validating, packaging, and rendering reports
• preserve each Codex or Claude execution as native JSONL while requiring only a shallow atomic result.json from the agent
• render a searchable, filterable, self-contained HTML report with explicit cross-reference anchors and collapsible raw evidence
• include a Proxy Protocol sample plan and automated tests

Validation

• git diff --cached --check
• python3 -m unittest discover -s tools/dstack-test/tests -v
• prek run --all-files
• reuse lint

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