Add parser to distinguish valid DAB variable interpolation from bash variables in scripts

[AryanBagade]

Changes

Add a parser to properly distinguish valid DAB variable interpolation from bash environment variables in scripts.

• Added FindAllInterpolationReferences() and HasValidDABPrefix() helper functions in libs/dyn/dynvar/ref.go
• Updated bundle/config/validate/scripts.go to allow valid DAB references (${var.x}, ${bundle.name}, etc.) while rejecting ambiguous patterns like ${FOO}
• Improved error messages with helpful suggestions (e.g., "use $FOO for env var or ${var.FOO} for bundle variable"

Why

Previously, all ${...} patterns were blocked in scripts with a generic error message. This made it impossible to use DAB variable interpolation in scripts, forcing users to rely on environment variables as a workaround.
With this change:

• ${var.foo}, ${bundle.name}, ${workspace.host}, etc. are now allowed and resolved
• $HOME and ${VAR:-default} (bash syntax) continue to work
• ${FOO} (ambiguous) shows a helpful error with suggestions

Fixes #4179

Tests

• Added unit tests in bundle/config/validate/scripts_test.go
• Added unit tests in libs/dyn/dynvar/ref_test.go
• Added acceptance test acceptance/bundle/run/scripts/valid-interpolation/
• Updated existing acceptance test acceptance/bundle/run/scripts/no-interpolation/
• All tests pass: go test ./libs/dyn/dynvar/... ./bundle/config/validate/...
• Lint passes: make lint (0 issues)

[AryanBagade]

@shreyas-goenka could you please review this PR.

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• PR number: 4197
• Commit SHA: 4ab5e20334d88f8358f23f88f268882f789038e8

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