exporter: Add retry logging to indexer connection (#4846)

## Motivation

When block exporters fail to connect to the indexer, they retry with
exponential backoff but provide no visibility into the retry process.
Operators cannot determine whether connection issues are transient
(retrying successfully) or persistent (about to fail). This lack of
observability makes it difficult to diagnose network connectivity issues
and indexer availability problems during deployments and operations.

## Proposal

Add comprehensive logging to the indexer connection retry logic:

- **WARNING level** on each retry attempt, including:
  - Current retry count and max retries remaining
  - Calculated backoff delay in milliseconds
  - Error that triggered the retry
- **ERROR level** when all retries are exhausted, clearly indicating:
  - Final retry count
  - Max retries configured
  - Error that caused final failure
  - Explicit statement that exporter task will exit

This provides operators with actionable telemetry to distinguish
transient network issues from persistent failures.

## Test Plan

1. Deploy network with intentionally unreachable indexer endpoint
2. Observe exporter logs show WARNING messages with increasing retry
counts and backoff delays
3. Wait for max retries to be exhausted
4. Verify ERROR message appears with clear failure indication
5. Confirm logs provide enough information to diagnose the connection
issue

## Release Plan

- These changes should be backported to the latest `testnet` branch,
then
    - be released in a validator hotfix.

Author: ndr-ds

linera-service/src/exporter/runloops/indexer/client.rs

@@ -98,10 +98,23 @@ impl IndexerClient {
                 }
                 Err(e) => {
                     if retry_count > self.max_retries {
+                        tracing::error!(
+                            retry_count,
+                            max_retries = self.max_retries,
+                            error = %e,
+                            "Failed to connect to indexer after exhausting all retries, exporter task will exit"
+                        );
                         return Err(ExporterError::SynchronizationFailed(e.into()));
                     }
 
                     let delay = self.retry_delay.saturating_mul(retry_count);
+                    tracing::warn!(
+                        retry_count,
+                        max_retries = self.max_retries,
+                        retry_delay_ms = delay.as_millis(),
+                        error = %e,
+                        "Failed to connect to indexer, retrying with exponential backoff"
+                    );
                     sleep(delay).await;
                     retry_count += 1;
                 }