Derive default Scala cluster runtime from DBR version

[rugpanov]

Changes

• Derive the default Scala classic-cluster Spark runtime from shared DBR and Scala version macros.
• Add regression coverage and a compact classic-template acceptance diff.
• Extend the template-version rule to cover the Scala build file and both runtime inputs.

Why

The DB Connect dependency already follows dbr_version, but the classic job template duplicated its runtime as a literal. Updating only the macro could therefore produce a client that cannot connect to the generated cluster.

Follow-up to #6407.

Tests

• go test ./libs/template -count=1
• go test ./acceptance -run TestAccept/bundle/templates/default-scala-classic (Terraform and direct variants)
• focused golangci-lint: 0 issues
• ./task checks
• ./task lint: all Go linters pass; Python lint could not download ruff because PyPI access is blocked
• ./task test: 9,786 tests pass; libs/patchwheel requires an unavailable Python 3.13 installation

This PR was written by Codex.

[eng-dev-ecosystem-bot]

Integration test report

Commit: 2e3f010

Run: 34223216447

	Env	💚​RECOVERED	✅​pass	🙈​skip	Time
💚​	aws windows	1	277	14	6:27
💚​	azure windows	1	276	14	7:09
💚​	gcp windows	1	277	14	8:42

	Test Name	aws windows	azure windows	gcp windows
💚​	TestAccept	💚​R	💚​R	💚​R

Top 4 slowest tests (at least 2 minutes):

duration	env	testname
3:39	azure windows	TestAccept
3:27	gcp windows	TestAccept
3:13	aws windows	TestAccept
2:26	gcp windows	TestFilerWorkspaceFilesExtensionsDelete