Fix: Assorted SDK issues and test coverage (Ajayi)

[pyper01]

This PR addresses the following issues:

• Closes Docs: docs/api.md's StreamBuilder reference omits ratePerSecond() and submit() #463: Docs: api.md's StreamBuilder reference omits ratePerSecond() and submit()
• Closes Test gap: no coverage for resolvePassphrase() (src/batch-tx.ts) #462: Test gap: no coverage for resolvePassphrase() (src/batch-tx.ts)
• Closes Test gap: no coverage for estimateRequiredFee() (src/soroban.ts) #461: Test gap: no coverage for estimateRequiredFee() (src/soroban.ts)
• Closes Test gap: no coverage for queryXlmBalance() (src/soroban.ts) #460: Test gap: no coverage for queryXlmBalance() (src/soroban.ts)

[Jaydbrown]

Closing as a duplicate — this diff is byte-identical to #526 (resolvePassphrase + soroban-rpc-helpers tests), which has been merged. Thanks for the contribution.