feat(sdk): add transaction retry with fee bumping and nonce management

[jerrymusaga]

Closes #171

Summary

• RetryPolicy config — maxAttempts, feeBumpMultiplier, maxFeeStroops, exponential backoff params; merged into bcForgeClientConfig
• Fee bumping — on tx_insufficient_fee, bumps the inclusion fee by the configured multiplier (default ×1.5) up to a hard cap (default 1 XLM)
• Nonce management — on tx_bad_seq, re-fetches the account (fresh sequence number) before rebuilding the transaction
• tx_too_late handling — rebuilds with a fresh timeout/ledger bounds and retries with back-off
• Transient RPC errors — exponential back-off retry on network-level failures (ECONNRESET, 503, 429, etc.)
• Fee estimation — estimateBaseFee() uses the RPC getFeeStats p99 inclusion fee as the starting fee for each write transaction
• Idempotency tracking — IdempotencyTracker records submitted tx hashes within a retry session so timed-out transactions can be re-checked before a fresh submission
• Error taxonomy — TxTooLateError, InsufficientFeeError, BadSequenceError, MaxRetriesExceededError, FeeLimitExceededError (all exported)

Test plan

• classifyTxError maps all three error codes + unknown/undefined cases
• bumpFee multiplies, rounds up, caps, and throws at the limit
• calculateBackoffDelay exponential growth capped at maxDelayMs
• isTransientError classifies RPCError and network error message patterns
• IdempotencyTracker records and queries submitted hashes
• executeWithRetry success on first attempt, fee-bump retry, back-off retries, exhausted attempts, fee cap, non-retryable errors, idempotency recording, custom initial fee
• All 42 tests pass (npm test)
• TypeScript strict mode (tsc --noEmit) — no errors

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