feat: add RPC failover support with per-request error recovery

[MayurK-cmd]

Closes #661

Summary

Implement multi-endpoint RPC failover to eliminate single point of failure.
Adds FailoverRpcClient for endpoint management, FailoverRpcCallWrapper for
per-request failover on 5xx/transport errors, CLI support for RPC_URLS
config, and maintains backward compatibility with single RPC_URL.

Changes

• FailoverRpcClient: manages multiple endpoints with round-robin rotation
• FailoverRpcCallWrapper: wraps RPC calls with error detection and retry logic
• CLI support for RPC_URLS comma-separated endpoints
• Updated main.rs to use failover factory function

Testing

• All 973 existing tests pass with no regressions
• 11 new unit tests for failover and error detection
• Tested error detection for 5xx, 429, connection, and timeout errors

[greptile-apps]

Greptile Summary

This PR adds configuration and library scaffolding for multiple Solana RPC endpoints, including endpoint selection, rotation state, retry classification, and CLI integration.

• Adds comma-separated RPC_URLS support while preserving RPC_URL and its localhost default.
• Introduces failover client and per-call retry wrapper types.
• Updates CLI startup and command paths to construct and share the selected RPC client.
• Fixes the endpoint fixture types and duplicate endpoint-vector consumption identified in prior reviews.

Confidence Score: 5/5

No accepted new issue from the changes since the previous review blocks merging.

The recent changes correct the CLI fixture type mismatch and endpoint-vector ownership problem without introducing a distinct new failure. All previous threads were manually resolved without explanation, so they are not treated as outstanding.

Important Files Changed

Filename	Overview
crates/cli/src/args.rs	Adds multi-endpoint argument resolution while preserving the legacy localhost default and corrects all new test fixtures.
crates/cli/src/main.rs	Constructs the configured RPC client once and shares its selected client across mutually exclusive command paths.
crates/lib/src/rpc.rs	Adds a factory and enum for single-endpoint and multi-endpoint RPC client configurations.
crates/lib/src/rpc_failover.rs	Adds shared endpoint storage and atomic round-robin rotation state.
crates/lib/src/rpc_failover_wrapper.rs	Adds per-call retry orchestration and retryable error classification without logging raw external errors.

_{Reviews (3): Last reviewed commit: "feat: add RPC failover support with per-..." | Re-trigger Greptile}