[AI-203] Standardize MCP tool input validation

[Kartikey-Malkani]

Jira: https://mifosforge.jira.com/browse/AI-203

Summary

• Add reusable python/tools/validation.py for shared MCP input validation
• Wrap all @mcp.tool() registrations in mcp_server.py with centralized pre-validation
• Short-circuit invalid requests before tool business logic / Fineract API calls
• Add focused tests in python/tests/test_validation.py

Validation Framework

• Validates common ID fields (*Id, *_id, id) as positive integers
• Validates common numeric fields (amount, principal, rate, period counters) as positive numbers
• Validates date-like fields as non-empty strings
• Returns early with consistent error structure on any validation failure

Error Response Format

{
  "error": "Invalid loan_id. It must be a positive integer.",
  "httpStatusCode": 400,
  "validation": {
    "field": "loan_id",
    "value": "invalid_value",
    "expected": "positive integer"
  }
}

Backward Compatibility

• Preserves all function signatures (no breaking changes)
• Uses conservative common validation rules only
• Existing callers continue to work unchanged

Verification

• Error checks: clean for all edited files
• Tests pass: 14 passed
• Command: python -m pytest tests/test_validation.py tests/test_loans.py -q

Closes AI-203

[IOhacker]

Use this ticket in the PR title: https://mifosforge.jira.com/browse/AI-203

[Kartikey-Malkani]

Thanks for the feedback! I’ve updated the PR to include the Jira ticket in the title and description.

Please let me know if anything else needs improvement or if it’s good to proceed.