Simplify Docker Compose naming to v2-only format

[cristianrgreco]

Summary

• Remove Compose v1-style project-prefix parsing and keep Compose v2 naming plus explicit container_name support.
• Simplify compose docs to document only the current <service-name>-<index> naming format.
• Rename compose fixture services from numeric names to service-a and service-b for clarity.
• Inline literal Compose v2 container names in compose tests and remove the unused composeContainerName helper.

Verification

• npm run format
• npm run lint
• npm test -- packages/testcontainers/src/container-runtime/clients/compose/parse-compose-container-name.test.ts packages/testcontainers/src/docker-compose-environment/docker-compose-environment.test.ts

Test Results

• 27 tests passed across 2 test files.

Semver Impact

• patch
• The change is not breaking: public APIs are unchanged and only unsupported Compose v1 naming compatibility code was removed.

Closes #840

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