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

cli-config.yaml.example

@@ -121,6 +121,18 @@ model:
 #   # Data policy: "allow" (default) or "deny" to exclude providers that may store data
 #   # data_collection: "deny"
 
+# =============================================================================
+# OpenRouter Response Caching (only applies when using OpenRouter)
+# =============================================================================
+# Cache identical API responses at the OpenRouter edge for free instant replays.
+# When enabled, identical requests (same model, messages, parameters) return
+# cached responses with zero billing. Separate from Anthropic prompt caching.
+# See: https://openrouter.ai/docs/guides/features/response-caching
+#
+# openrouter:
+#   response_cache: true         # Enable response caching (default: true)
+#   response_cache_ttl: 300      # Cache TTL in seconds, 1-86400 (default: 300)
+
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