fix(payment-methods): add ph code validation in customer create flow

[prasunna09]

Type of Change

• Bugfix
• New feature
• Enhancement
• Refactoring
• Dependency updates
• Documentation
• CI/CD

Description

Additional Changes

• This PR modifies the API contract
• This PR modifies the database schema
• This PR modifies application configuration/environment variables

Motivation and Context

How did you test it?

Setup:

1. Create merchant account
2. Create API key for that merchant

Test 1 — valid phone_country_code ("+1"):

curl -s -X POST http://localhost:8080/customers \
  -H "Content-Type: application/json" \
  -H "api-key: dev_PYPUYZwF1HNg2gZeMYUy4qINUEKVZvJjNRK9OxqmIK67k84vnvTNGQS71Tty6kAw" \
  -d '{
    "name": "Jon Test",
    "email": "jontest@example.com",
    "phone": "9123456789",
    "phone_country_code": "+1"
  }'

HTTP/1.1 200 OK

{"customer_id":"cus_6dxPT61GjFgDhhs0caJ0","name":"Jon
Test","email":"jontest@example.com","phone":"9123456789","phocription":null,"address":null,"created_at":"2026-08-18T18:57:39.168Z","metadata":null,"default_payment_method_id":null,"tax_registration_id":null,"document_details":null}

Test 2 — invalid phone_country_code ("United States", the exact value from the bug report):

curl -s -X POST http://localhost:8080/customers \
  -H "Content-Type: application/json" \
  -H "api-key: dev_PYPUYZwF1HNg2gZeMYUy4qINUEKVZvJjNRK9OxqmIK
  -d '{
    "name": "Jon Test 2",
    "email": "jontest2@example.com",
    "phone": "9123456789",
    "phone_country_code": "United States"
  }'

HTTP/1.1 400 Bad Request (previously 500)

{"error":{"type":"invalid_request","message":"Invalid value p must be a valid country calling code (e.g. \"+1\"), got\"United States\"","code":"IR_07"}}

Confirmed via the server log too — the invalid request is handled as a normal BadRequest, not a panic or unhandled DB error.

Checklist

• I formatted the code cargo +nightly fmt --all
• I addressed lints thrown by cargo clippy
• I reviewed the submitted code
• I added unit tests for my changes where possible

[semanticdiff-com]

Review changes with  

Changed Files

File	Status
crates/common_utils/src/validation.rs	0% smaller
crates/router/src/core/customers.rs	0% smaller

[su-shivanshmathur]

Should this go to consts.rs ?