feat(openrouter): add response caching support (#19132)

Enable OpenRouter's response caching feature (beta) via X-OpenRouter-Cache
headers. When enabled, identical API requests return cached responses for
free (zero billing), reducing both latency and cost.

Configuration via config.yaml:
  openrouter:
    response_cache: true       # default: on
    response_cache_ttl: 300    # 1-86400 seconds

Changes:
- Add openrouter config section to DEFAULT_CONFIG (response_cache + TTL)
- Add build_or_headers() in auxiliary_client.py that builds attribution
  headers plus optional cache headers based on config
- Replace inline _OR_HEADERS dicts with build_or_headers() at all 5 sites:
  run_agent.py __init__, _apply_client_headers_for_base_url(), and
  auxiliary_client.py _try_openrouter() + _to_async_client()
- Add _check_openrouter_cache_status() method to AIAgent that reads
  X-OpenRouter-Cache-Status from streaming response headers and logs
  HIT/MISS status
- Document in cli-config.yaml.example
- Add 28 tests (22 unit + 6 integration)

Ref: https://openrouter.ai/docs/guides/features/response-caching

hermes_cli/config.py

@@ -644,6 +644,18 @@ DEFAULT_CONFIG = {
         "cache_ttl": "5m",
     },
 
+    # OpenRouter-specific settings.
+    # response_cache: enable OpenRouter response caching (X-OpenRouter-Cache header).
+    #   When enabled, identical requests return cached responses for free (zero billing).
+    #   This is separate from Anthropic prompt caching and works alongside it.
+    #   See: https://openrouter.ai/docs/guides/features/response-caching
+    # response_cache_ttl: how long cached responses remain valid, in seconds (1-86400).
+    #   Default 300 (5 minutes). Only used when response_cache is enabled.
+    "openrouter": {
+        "response_cache": True,
+        "response_cache_ttl": 300,
+    },
+
     # AWS Bedrock provider configuration.
     # Only used when model.provider is "bedrock".
     "bedrock": {