Fix release smoke and startup default export

[haasonsaas]

Summary

• export startup.DefaultShutdownTimeout for public consumers mirrored out of platform
• keep release smoke module caches outside the temporary module root
• run go mod tidy before the clean-cache smoke build and make cleanup tolerate read-only module cache files

Test Plan

• go test ./startup
• go test ./...
• actionlint .github/workflows/release.yml
• git diff --check
• local clean-cache smoke with github.com/evalops/service-runtime@v0.1.49 importing natsbus

[cursor]

PR Summary

Low Risk
Low risk: CI smoke-test workflow tweaks and a small exported constant with a simple unit test; no runtime logic changes beyond build/release automation.

Overview
Fixes the release workflow’s clean-cache smoke test by isolating the temp module workdir from GOMODCACHE/GOCACHE, running go mod tidy before go build, and making temp cleanup tolerate read-only cache files.

Exports startup.DefaultShutdownTimeout (10s) for consumers and adds a unit test to lock in the default value.

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