Copy Redpanda runtime assets during build - #1357

Summary

• copy Redpanda runtime assets during postbuild so normal workspace builds produce usable output
• keep prepack focused on package metadata files
• document that runtime assets required by compiled code must be copied during the normal build lifecycle

Verification

• npm run build --ignore-scripts --workspace packages/testcontainers -- --project tsconfig.json
• npm run build --ignore-scripts --workspace packages/modules/redpanda -- --project tsconfig.json --noEmit
• npm run build --workspace packages/modules/redpanda
• npm_config_cache=/private/tmp/testcontainers-node-npm-cache npm pack --dry-run --json --workspace @testcontainers/redpanda
• npm test -- packages/modules/redpanda/src/redpanda-container.test.ts
• npm run format
• npm run lint
• npm run check-compiles
• git diff --check

Test results

• Red: a normal Redpanda workspace build left build/assets absent before this change.
• Green: a normal Redpanda workspace build now creates build/assets/bootstrap.yaml and build/assets/redpanda.yaml.hbs.
• Green: the compile-only invocation keeps CI-provided flags attached to tsc and does not run postbuild.
• Green: the dry-run package archive includes both runtime assets.
• Green: the focused Redpanda suite passed (4 passed).

Compatibility

This is a backward-compatible bug fix. The public API is unchanged. Published packages retain the same runtime assets, while local workspace builds now produce the complete runtime output without relying on prepack.