feat: add HTTP subscription support via polling

[smartprogrammer93]

Summary

Implements HTTP subscription support via polling, allowing HTTP providers to participate in block subscriptions alongside WebSocket providers. This addresses issue #23.

Changes

Core Feature (http-subscription feature flag)

• HttpPollingSubscription: New subscription backend that polls eth_getBlockByNumber("latest") at configurable intervals
• Block deduplication: Tracks last seen block number to prevent duplicate emissions
• Seamless integration: Works transparently with existing RobustSubscription API - callers use the same subscribe_blocks() and recv() interface

Builder API

RobustProviderBuilder::new(ws_provider)
    .fallback(http_provider)
    .allow_http_subscriptions(true)  // Enable HTTP polling for subscriptions
    .poll_interval(Duration::from_secs(12))  // Polling frequency
    .build()

Failover Behavior

• HTTP providers can now participate in subscription failover chains
• Mixed WS/HTTP failover works correctly (WS → HTTP and HTTP → WS)
• HTTP reconnection validates provider reachability before claiming success (prevents "reconnecting" to dead providers)
• Configuration (poll_interval, call_timeout, buffer_capacity) properly propagates during failover

Files Added/Modified

File	Description
src/robust_provider/http_subscription.rs	New HTTP polling subscription implementation
src/robust_provider/subscription.rs	SubscriptionBackend enum, HTTP failover logic
src/robust_provider/builder.rs	poll_interval(), allow_http_subscriptions() methods
src/robust_provider/provider.rs	HTTP subscription creation in subscribe_blocks()
tests/http_subscription.rs	Comprehensive integration tests
tests/rpc_failover.rs	RPC failover integration tests

Test Coverage

• Basic HTTP subscription flow
• Block deduplication
• Failover: WS → HTTP, HTTP → WS
• Timeout-triggered failover with multiple fallbacks
• Provider exhaustion behavior
• Configuration propagation from builder
• HTTP reconnect validation
• Stream conversion
• Error handling and recovery

Usage

// Enable the feature
// Cargo.toml: robust-provider = { version = "...", features = ["http-subscription"] }

let provider = RobustProviderBuilder::new(ws_provider)
    .fallback(http_only_provider)
    .allow_http_subscriptions(true)
    .poll_interval(Duration::from_secs(12))
    .subscription_timeout(Duration::from_secs(60))
    .build()
    .await?;

// Use exactly like before - HTTP polling is transparent
let mut subscription = provider.subscribe_blocks().await?;
while let Ok(block) = subscription.recv().await {
    println!("Block {}", block.number);
}

Breaking Changes

None. Feature is behind http-subscription flag and disabled by default.

Closes #23