FINERACT-xxxx: Add equals() and hashCode() to LoanTransaction

[Harshdhall01]

Description

LoanTransaction was missing equals() and hashCode() implementations,
flagged by a long-standing TODO comment in the code:

// TODO missing hashCode(), equals(Object obj), but probably OK as long as
// this is never stored in a Collection.

The parent class AbstractPersistableCustom intentionally omits these methods
(documented in its own Javadoc). Added id-based implementations consistent with
the pattern already used in LoanDisbursementDetails, ensuring correct behavior
if LoanTransaction is ever used in hash-based collections.

Changes

• Removed stale TODO comment
• Added equals() based on entity id with null-safety for unsaved entities
• Added hashCode() based on entity id
• Added missing java.util.Objects import

Note: JIRA ticket will be linked once Apache JIRA account approval is received.

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I am exploring this codebase as part of my DMP 2026 (C4GT) application for the
"Dynamic Pricing of Micro-loans Using Reinforcement Learning" project under
openMF/Mifos. The loan module is directly relevant as it contains the core
transaction logic that my RL environment will model for dynamic pricing decisions.

Checklist

• Write the commit message as per our guidelines
• Acknowledge that we will not review PRs that are not passing the build
• Create/update unit or integration tests for verifying the changes made
• Follow our coding conventions
• Add required Swagger annotation and update API documentation
• This PR must not be a "code dump"