Build SDK fallback chain

[Kingsman-99]

Label: complexity: high
Points: 200

Description

The current failover logic tries endpoints in a fixed order without logging why each failed. This issue adds a FallbackChain that tries a sequence of RPC endpoints, logs each failure with reason, and returns the first successful result.

Technical Context

Involves a new src/fallbackChain.ts. FallbackChain accepts an ordered array of RPC URLs. For each request, try URLs in order. On failure, log { url, error, attemptMs } and try next. On all failures, throw a FallbackExhaustedError containing all attempt logs. Export FallbackChain and FallbackExhaustedError.

Acceptance Criteria

• FallbackChain class exported from src/index.ts
• Tries endpoints in provided order
• Each failure logged with URL, error message, and duration
• FallbackExhaustedError thrown with all attempt logs when all fail
• Test with 2 failing URLs verifies error contains both attempt logs
• All existing tests pass
• TypeScript strict mode — zero any types

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