security: enforce strict input validation and payload bounds

[SurbhiAgarwal1]

Description

This PR implements strict input validation and payload bounds within the tokens.Service.Append method to protect the system against resource exhaustion and potential Denial of Service (DoS) attacks from malformed or oversized finality data.

Changes

• ValidationConfig: Introduced a configurable validation layer with safe default limits:
  • MaxTokenPayloadSize: 1MB (covers TokenOnLedger and TokenOnLedgerMetadata)
  • MaxTokenOutputsPerTx: 1000 outputs
  • MaxBulkDeleteSize: 10,000 IDs
  • MaxWalletIDSize: 1024 bytes
  • MaxOwnerRawSize: 2048 bytes
  • MaxIssuerRawSize: 2048 bytes
• AppendRequest: Refactored the internal data flow to use a request-based structure for validated processing.
• Strict Validation Logic: Implemented validateAppendRequest which performs in-memory checks before any database transactions or storage operations are initiated.
• Enhanced Testing: Added a comprehensive test suite in token/services/tokens/validation_test.go covering edge cases, limit violations, and structural validation.

Verification Results

• Unit Tests: All tests in token/services/tokens passed successfully.
• Linting: Code is clean according to golangci-lint (v2) standards, including nlreturn and testifylint compliance.

Fixes: #1608

[HayimShaul]

Looks good to me.
@adecaro do you want to have another quick look?

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