fix: Restore result.metrics.tokens in provider README quick-start examples

LDAIMetricSummary's token field is `tokens`, not `usage` (verified at
packages/sdk/server-ai/src/ldai/tracker.py — the `@property def tokens`
on LDAIMetricSummary). The earlier change to `.usage` was a regression
based on an unverified asymmetry claim; reverting.

Refs AIC-2383

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: jsonbailey

packages/ai-providers/server-ai-langchain/README.md

@@ -126,7 +126,7 @@ model = await ai_client.create_model("ai-config-key", context)
 if model:
     result = await model.run("Explain feature flags.")
     # Metrics are tracked automatically; access them via result.metrics
-    print(result.metrics.usage)
+    print(result.metrics.tokens)
 ```
 
 ### Static Utility Methods

packages/ai-providers/server-ai-openai/README.md

@@ -112,7 +112,7 @@ model = await ai_client.create_model("ai-config-key", context)
 if model:
     result = await model.run("Explain feature flags.")
     # Metrics are tracked automatically; access them via result.metrics
-    print(result.metrics.usage)
+    print(result.metrics.tokens)
 ```
 
 ### Static Utility Methods