[runtime] Harden dual-version route test boot diagnostics

[GsCommand]

Motivation

• Make the versioned-routes test deterministic and diagnosable when the test-spawned server fails to start under CI.

Description

• Resolve and spawn server.mjs via an absolute path from the test file so startup does not depend on ambiguous cwd behavior.
• Align the test's signer environment with the runtime's production env contract by using only RECEIPT_SIGNING_PRIVATE_KEY_PEM_B64, RECEIPT_SIGNING_PUBLIC_KEY_B64, and RECEIPT_SIGNER_ID.
• Capture child stdout/stderr and include those logs in thrown boot errors to surface real startup failures instead of a generic timeout.
• Remove an unused test-only derived kid value from the generated key material.

Testing

• Ran npm run check and the server file syntax check completed successfully.
• Ran npm run test:unit and all unit tests passed.
• Ran npm test (unit tests plus tests/smoke.mjs) and the full test suite, including smoke, passed.

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Codex Task