feat: implement minimum invoice amount from protocol config with test…

[devJaja]

Implement Minimum Invoice Amount from Protocol Config

Summary

Implements minimum invoice amount validation from protocol config in store_invoice and
upload_invoice. Invoices below the configured minimum are rejected with InvalidAmount error.

Changes

Core Implementation

• src/lib.rs: Updated store_invoice to use
  protocol_limits::ProtocolLimitsContract::validate_invoice()
• src/protocol_limits.rs: Existing validate_invoice() already checks
  amount < limits.min_invoice_amount
• src/verification.rs: Already validates via protocol limits (no changes needed)

Tests

Added 6 new test cases:

• test_invoice_creation_below_minimum_amount - Rejects 999 < 1000
• test_invoice_creation_at_minimum_amount - Accepts 1000
• test_upload_invoice_below_minimum_amount - Tests upload path
• src/test_min_invoice_amount.rs - Standalone validation tests (3 cases)

Documentation

• docs/contracts/limits.md: Added comprehensive "Numeric Value Limits" section with validation
  flow, admin configuration, and security considerations

Bug Fixes

• src/settlement.rs - Fixed incorrect import
• src/test_currency.rs - Fixed missing closing brace
• src/test_queries.rs - Fixed incomplete function calls

Configuration

• Default (test): 1,000
• Default (production): 1,000,000 (1 token with 6 decimals)
• Admin configurable: Yes, via set_protocol_limits()

Security

• ✅ Single source of truth in protocol_limits.rs
• ✅ Admin-only configuration
• ✅ Validation at all entry points
• ✅ Immutable after invoice creation

Testing

• ✅ 6 new test cases covering edge cases
• ✅ Library builds successfully
• ✅ No new warnings introduced

Requirements Met

• ✅ Secure, tested, and documented
• ✅ Single source of truth for minimum amount
• ✅ Smart contracts only (Soroban/Rust)
• ✅ Uses protocol config validation
• ✅ Rejects with InvalidAmount error
• ✅ Documents default and admin update

Notes

• Pre-existing WASM size issue (326KB > 256KB) unrelated to this PR
• Pre-existing test compilation errors unrelated to this PR
• Implementation adds minimal code

Closes #484