Add unit tests for graceful-shutdown service + Add unit tests for request-router service

[ykargeee-bit]

Closes #1275
…

Summary

Adds focused unit test suites for two previously untested common services, bringing their public APIs under verified coverage.

What changed

File	Description
src/common/request-router.service.spec.ts	8 tests covering selectTarget — single/multiple targets, empty array, weight boundaries, fractional weights, and floating-point edge cases
src/common/services/graceful-shutdown.service.spec.ts	19 tests covering registerShutdownPhase, shutdown, isShutdownInProgress, getShutdownStatus, and onModuleDestroy — including phase ordering, error/timeoute resilience, force-exit timer behavior, and dedup guard
package.json / package-lock.json	Added ts-jest as a dev dependency (required by the existing Jest config but was missing from node_modules)
Closes #1277

Motivation

Both services had no accompanying *.spec.ts files, making their behaviour unverified and easy to regress. These tests lock down the public API surface and document edge cases (e.g. non-numeric SHUTDOWN_TIMEOUT_MS results in NaN).

Test results

Closes #1276

Notes

• The SHUTDOWN_TIMEOUT_MS non-numeric env var case surfaces a latent bug (parseInt returns NaN with no fallback). The test documents this; a follow-up can add a guard.
• FORCE_EXIT_ON_TIMEOUT=false is set in the test environment to prevent process.exit(1) from killing the test runner.

Closes #1278

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