fix(router): validate each field for migration request body

[prasunna09]

Type of Change

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

Description

#6300 has inconsistent response due to no validations for each field in migration api request body.

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?

test case - test migration api with empty fields. req -

curl --location 'http://localhost:8080/payment_methods/migrate' \
--header 'Content-Type: application/json' \
--header 'Accept: application/json' \
--header 'api-key: api-key' \
--data '{
    "merchant_id": "merchant_id",
    "card": {
        "card_number": "card_number",
        "card_exp_month": "12",
        "card_exp_year": "21",
        "card_holder_name": "joseph Doe"
    },
    "customer_id": "customer_id",
    "network_transaction_id": "",
    "payment_method": "card",
    "connector_mandate_details": {},
    "network_token": {
        "network_token_data": {
            "network_token_number": "  ",
            "network_token_exp_month": "12",
            "network_token_exp_year": "21"
        },
        "network_token_requestor_ref_id": ""
    }
}'

Checklist

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