feat(indexer): add Stellar RPC connectivity check to health endpoint

[sulaimonifeoluwa4-blip]

closes #78

Description

This PR adds a downstream Stellar RPC connectivity check to our health tracking endpoint. The health payload now verifies that our interface can actively establish communication with the Stellar network cluster without creating a cascading point-of-failure if the provider drops temporarily.

Changes

• 🩺 RPC Health Verification: Added an async probe checking server.getLatestLedger() inside the main health router context.
• ⏱️ Timeout Guard: Wrapped the network network call with a strict 5-second timeout execution boundary.
• 📦 Payload Extension: Enriched the API JSON response to supply rpc.connected indicators alongside optional rpc.latestLedger sequence heights.
• 🛡️ Fault Tolerance: Out-of-bounds networks or remote exceptions drop the connection flag gracefully instead of throwing unhandled 500 app panics.

Acceptance Criteria Verification

• RPC Tracking: Appends { rpc: { connected: boolean, latestLedger?: number } } into the active telemetry.
• Timeout Enforcement: Slashing connection attempts at the 5-second limit cleanly.
• Isolation: Top-level global service status remains uninhibited during targeted connection drops.

How to Test

1. Make a payload request to your local health route:

   curl http://localhost:3000/health## Description

Ref #78. This PR adds a downstream Stellar RPC connectivity check to our health tracking endpoint. The health payload now verifies that our interface can actively establish communication with the Stellar network cluster without creating a cascading point-of-failure if the provider drops temporarily.

Changes

• 🩺 RPC Health Verification: Added an async probe checking server.getLatestLedger() inside the main health router context.
• ⏱️ Timeout Guard: Wrapped the network network call with a strict 5-second timeout execution boundary.
• 📦 Payload Extension: Enriched the API JSON response to supply rpc.connected indicators alongside optional rpc.latestLedger sequence heights.
• 🛡️ Fault Tolerance: Out-of-bounds networks or remote exceptions drop the connection flag gracefully instead of throwing unhandled 500 app panics.

Acceptance Criteria Verification

• RPC Tracking: Appends { rpc: { connected: boolean, latestLedger?: number } } into the active telemetry.
• Timeout Enforcement: Slashing connection attempts at the 5-second limit cleanly.
• Isolation: Top-level global service status remains uninhibited during targeted connection drops.

How to Test

1. Make a payload request to your local health route:

   curl http://localhost:3000/health

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