feat: add /utils/validate-hash and /utils/network-passphrase endpoints (#216, #219)

[3m1n3nc3]

Closes #216
Closes #219

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Summary

Adds two local (no Horizon call) developer-utility endpoints under /utils:

• GET /utils/validate-hash?hash=... (Transaction Hash Format Validator Endpoint #216) — validates whether a string is a
  correctly formatted Stellar transaction hash. Valid = exactly 64 lowercase
  hexadecimal characters. Returns { input, isValid, reason }, with reason
  explaining wrong length or invalid characters (uppercase hex is rejected).
• GET /utils/network-passphrase (Network Passphrase Resolver Endpoint #219) — returns the network passphrase for
  the currently configured network. Returns { network, passphrase }, resolving
  STELLAR_NETWORK=mainnet → Networks.PUBLIC and otherwise → Networks.TESTNET
  (default testnet).

Both follow the existing validate-account convention and are registered in the
root API endpoint listing.

Example responses

// GET /utils/validate-hash?hash=3389e9f0...c8889
{ "success": true, "data": { "input": "3389e9f0...c8889", "isValid": true, "reason": null } }

// GET /utils/network-passphrase  (testnet)
{ "success": true, "data": { "network": "testnet", "passphrase": "Test SDF Network ; September 2015" } }

Acceptance criteria

#216

• GET /utils/validate-hash?hash=... returns { input, isValid, reason }
• Valid hash is exactly 64 lowercase hexadecimal characters
• Returns reason for wrong length / invalid characters
• Tests added

#219

• GET /utils/network-passphrase returns { network, passphrase }
• Correct passphrase for both testnet and mainnet based on STELLAR_NETWORK
• Tests added

Testing

• tests/utils.validateHash.test.js (7 tests) and
  tests/utils.networkPassphrase.test.js (3 tests) — all 10 pass.
• ESLint clean on changed files.

[Sulex45]

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[3m1n3nc3]

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Done