fix(builder): validate C... contract recipient addresses in StreamBuilder (closes #609)

[Ranjeet2063]

Summary

Closes #609

Ensures full parity with #587 by validating Soroban contract ID (C...) recipient addresses using isValidAddress in StreamBuilder and ConduitBatcher.

Key Changes

• src/builder.ts:
  • Imported and used isValidAddress for StreamBuilder._validateAddress and validatePayload address validation.
  • Ensures valid Soroban contract IDs (C...) are accepted and cleanly encoded into contract arguments via Address.toScVal().
  • Rejects malformed or invalid contract IDs with clear, actionable error messages.
• src/tests/builder-contract-recipient.test.ts:
  • Added unit tests validating:
    1. Valid C... contract recipient acceptance in StreamBuilder.build().
    2. Correct encoding as ScAddressTypeContract inside StreamBuilder.toContractArgs().
    3. Rejection of malformed checksum C... addresses.
    4. Rejection of empty or whitespace recipient strings.
    5. Validation of contract recipients within ConduitBatcher.execute.

Verification

• 5/5 new unit tests pass in src/tests/builder-contract-recipient.test.ts.
• All existing builder tests (55/55) pass without regression.
• npm run build generates clean ESM and CJS bundles.

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