FINERACT-2511: Return 400 for invalid dateFormat on client create/update

[oluexpert99]

• Validate dateFormat in ClientDataValidator (create and update) so invalid patterns (e.g. "02 February 2026") are rejected with 400 instead of 500.
• Add @DateFormat constraint and DateFormatValidator in fineract-validation; ClientDataValidator uses DateFormatValidator.isValidPattern() for reuse.
• Annotate BusinessDateUpdateRequest.dateFormat with @DateFormat.
• Add ClientDataValidatorTest and DateFormatValidationTest.

Description

Describe the changes made and why they were made. (Ignore if these details are present on the associated Apache Fineract JIRA ticket.)

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[oluexpert99]

@adamsaghy , This is a cleaner version