fix: validate Stellar public keys across DTOs and escrow service (#60)

[ghzhost]

Description

This PR resolves #60 by adding cryptographic and format validation for Stellar public keys (StrKey Ed25519 format) across all request DTOs and internal EscrowService methods.

Changes

1. Reusable Validator (IsStellarAddress & isValidStellarAddress):
   • Implemented in src/common/validators/stellar-address.validator.ts backed by @stellar/stellar-sdk's StrKey.isValidEd25519PublicKey.
   • Validates that addresses start with G, are 56 characters, valid base32, and satisfy CRC16 checksum verification.
2. DTO Layer Hardening:
   • Applied @IsStellarAddress() to:
     • FundEscrowDto.funderAddress
     • ReleaseEscrowDto.recipientAddress
     • SplitRecipientDto.recipientAddress
     • DepositDto.funderAddress (src/maintenance-pool/maintenance-pool.controller.ts)
     • AssignRewardDto.recipientAddress (src/maintenance-pool/maintenance-pool.controller.ts)
     • FundBountyDto.funderAddress (src/bounties/bounties.controller.ts)
     • FundMilestoneDto.funderAddress & ResolveIssueDto.recipientAddress (src/milestones/milestones.controller.ts)
     • SetStellarAddressDto.stellarAddress (src/users/users.controller.ts)
3. Service Layer Defense-in-Depth:
   • Updated EscrowService.fund, release, releasePartial, and assertValidSplits to validate Stellar addresses prior to database or Soroban contract invocations.
4. Test Coverage:
   • Added src/common/validators/stellar-address.validator.spec.ts testing unit validity (good keys, empty string, malformed strings, checksum failures) and HTTP boundary rejection with 400 Bad Request.
   • Updated src/escrow/escrow.service.spec.ts with valid Ed25519 test public keys and added tests for invalid funder and recipient address rejections.
   • Updated test mock in test/mocks/stellar-sdk.mock.js to include StrKey.isValidEd25519PublicKey.

Closes #60

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