Address review feedback: enhance type safety and test coverage

[Copilot]

Addressed 4 issues raised by automated code review on the autonomous refactoring PR.

Changes

• templateParser.ts: Extended getContextValue to handle number/boolean primitives by converting to string, not just string types. Ensures robust nested property access ({{ userData.port }}) even if type coercion occurs upstream.

• helpers.ts: Added 19 unit tests for getErrorMessage and detectCollisions functions to match coverage standards of other backend modules.

• config.ts: Changed VALID_BACKENDS type from readonly string[] to readonly ['traefik'] to preserve literal type information and maintain type safety.

// Before: loses literal type
const VALID_BACKENDS: readonly string[] = ['traefik'] as const;

// After: maintains literal type 'traefik'
const VALID_BACKENDS: readonly ['traefik'] = ['traefik'];

No changes to logger.ts - the optimization (computing bytes only when truncating) is correct as-is.

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[stonegray]

LGTM