FINERACT-2649: Migrate Tier 1 loan integration tests to Feign client

[DeathGun44]

Description

Migrates 5 Tier 1 loan integration tests from BaseLoanIntegrationTest (REST-assured) to FeignLoanTestBase (Feign client).

Migrated tests

• LoanPrepayAmountTest
• LoanProductWithChargeOffBehaviourTest
• LoanInterestRateFrequencyTest
• LoanDueCalculationTest
• FixedLengthLoanProductIntegrationTest

Infrastructure additions

• Added retrieveLoanProduct and updateLoanProduct (typed) methods to FeignLoanHelper
• Added updateLoanProduct(Long, String) raw JSON overload in FeignLoanTestBase for explicit null values (the NON_NULL ObjectMapper prevents typed null serialization)
• Added REST-assured requestSpec/responseSpec to FeignLoanTestBase for raw JSON edge cases
• Added fixedLengthLoanProduct template to LoanProductTemplates
• Added payment allocation helpers and loan request builders to LoanRequestBuilders
• Added schedule validation helpers to LoanTestValidators

Bug fixes

• FeignClientHelper.createClient() - use fixed past activation date (04 March 2011) matching ClientHelper.DEFAULT_DATE instead of real system date
• FeignBusinessDateHelper - date format fixed to use DATETIME_PATTERN (dd MMMM yyyy)

Checklist

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• Add required Swagger annotation and update API documentation at fineract-provider/src/main/resources/static/legacy-docs/apiLive.htm with details of any API changes
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