Add debug logging for connector-service feature flags request and response (#1462)

## Summary

Adds debug-level logging for the connector-service feature flags
endpoint:

1. **Request**: Logs the endpoint URL and User-Agent header before the
HTTP call
2. **Response**: Logs the full JSON response body on success

This helps diagnose feature flag rollout issues — e.g., which version
the driver reports to connector-service, which flags are returned, and
whether version-gating is filtering expected flags.

Example output at `LogLevel=6`:
```
Fetching feature flags from connector-service: endpoint=https://host/api/2.0/connector-service/feature-flags/OSS_JDBC/3.4.1, User-Agent=DatabricksJDBC/3.4.1 ...
Feature flags response from connector-service (endpoint=...): {"flags":[{"name":"...enableTelemetryForJdbc","value":"true"},...],"ttlSeconds":900}
```

NO_CHANGELOG=true

## Test plan
- [x] DatabricksConnectionContextTest — 131 tests pass

This pull request was AI-assisted by Isaac.

Signed-off-by: Gopal Lal <gopal.lal@databricks.com>

Author: gopalldb

src/main/java/com/databricks/jdbc/common/safe/DatabricksDriverFeatureFlagsContext.java

@@ -126,6 +126,10 @@ void fetchAndSetFlagsFromServer(IDatabricksHttpClient httpClient, HttpGet reques
             featureFlags.put(flag.getName(), flag.getValue());
           }
         }
+        LOGGER.debug(
+            "Feature flags from connector-service: endpoint={}, {}",
+            request.getURI(),
+            featureFlagsResponse);
 
         Integer ttlSeconds = featureFlagsResponse.getTtlSeconds();
         if (ttlSeconds != null) {

src/main/java/com/databricks/jdbc/common/safe/FeatureFlagsResponse.java

@@ -20,6 +20,19 @@ public Integer getTtlSeconds() {
     return ttlSeconds;
   }
 
+  @Override
+  public String toString() {
+    StringBuilder sb = new StringBuilder("FeatureFlagsResponse{flags=[");
+    if (flags != null) {
+      for (int i = 0; i < flags.size(); i++) {
+        if (i > 0) sb.append(", ");
+        sb.append(flags.get(i));
+      }
+    }
+    sb.append("], ttlSeconds=").append(ttlSeconds).append("}");
+    return sb.toString();
+  }
+
   @JsonIgnoreProperties(ignoreUnknown = true)
   public static class FeatureFlagEntry {
     @JsonProperty("name")
@@ -35,5 +48,10 @@ public String getName() {
     public String getValue() {
       return value;
     }
+
+    @Override
+    public String toString() {
+      return name + "=" + value;
+    }
   }
 }