T5(customer): add E2E smoke test script for Customer microservice

[devin-ai-integration]

Summary

Adds tests/customer-smoke-test.sh — a bash smoke test that validates the Customer microservice E2E through the API gateway.

Test flow:

1. Waits for customer service (:5002) and gateway (:5000) with 30s retry loops
2. GET /healthz directly on customer service → expects 200
3. GET /api/customers through gateway without token → expects 401
4. Generates a valid JWT (PyJWT, shared dev secret) and POST /api/customers → expects 201
5. GET /api/customers with JWT → expects 200, verifies created customer appears in response

URLs are configurable via GATEWAY_URL / CUSTOMER_URL env vars (default localhost:5000 / localhost:5002). Exit code 0 on all-pass, 1 on any failure.

Part of the Customer microservice carve-out stacked PR chain (T5/T5).

Link to Devin session: https://partner-workshops.devinenterprise.com/sessions/df4fd98794c34738bf5a70c588276dcb
Requested by: @mbatchelor81

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

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