feat(prompt-cache): cross-session 1h prefix cache for Claude on Anthropic / OpenRouter / Nous Portal (#23828)

Cuts input cost for first-turn Claude requests by ~85-90% on subsequent
sessions within an hour. Tools array (~13k tokens for default toolset) +
stable system prefix (~5-8k tokens) get a 1h cache_control marker; the
volatile suffix (memory, USER profile, timestamp, session id) sits in a
separate non-cached block at the end so it doesn't poison the cross-session
prefix when it changes.

Provider gate: Claude on native Anthropic (incl. OAuth subscription),
OpenRouter, and Nous Portal (which proxies to OpenRouter). All other
providers keep today's system_and_3 layout unchanged.

Layout (4 cache_control breakpoints, Anthropic max):
  1. tools[-1]              -> 1h (cross-session)
  2. system content[0]      -> 1h (cross-session, stable prefix)
  3. messages[-2]           -> 5m (within-session rolling)
  4. messages[-1]           -> 5m (within-session rolling)

Within-session rolling shrinks from 3 messages to 2 to free the breakpoint
budget. On Claude with realistic tool loadouts the long-lived tier carries
the bulk of cross-session value anyway.

System prompt is now always assembled cache-friendly: stable identity /
guidance / skills / platform hints first, then session-stable context
files (AGENTS.md, .cursorrules), then per-call volatile content. Old
single-string callers see the same logical content (same join order),
just reordered so volatile lives at the end.

Config knobs (defaults shown):
  prompt_caching:
    cache_ttl: "5m"           # rolling-window TTL (unchanged)
    long_lived_prefix: true    # opt-out switch
    long_lived_ttl: "1h"       # cross-session prefix TTL

Live E2E (tests/agent/test_prompt_caching_live.py, gated on
OPENROUTER_API_KEY) on anthropic/claude-haiku-4.5 with default toolset:
  Call 1 (cold):              cache_write=13,415  cache_read=0
  Call 2 (NEW agent + msg):   cache_write=391     cache_read=13,025
  Cross-session reuse:        97.09%

Implementation:
* agent/prompt_caching.py: new apply_anthropic_cache_control_long_lived()
  + mark_tools_for_long_lived_cache(); existing apply_anthropic_cache_control()
  preserved verbatim for the fallback path.
* agent/anthropic_adapter.py: convert_tools_to_anthropic() now forwards
  cache_control onto each Anthropic-format tool dict.
* run_agent.py: _build_system_prompt_parts() returns the 3-tier dict;
  _build_system_prompt() joins them (backward compatible).
  _supports_long_lived_anthropic_cache() policy added next to the existing
  _anthropic_prompt_cache_policy() (which now also recognises Nous Portal
  Claude — pre-existing gap fixed in passing).
  _build_api_kwargs() resolves tools_for_api once and propagates the
  marker through all four build paths (anthropic_messages, bedrock,
  codex_responses, profile/legacy chat completions).
  Long-lived flag plumbed into the runtime snapshot/restore + model-switch
  + fallback-promotion paths.

Tests:
* tests/agent/test_prompt_caching.py: +8 tests (TestMarkToolsForLongLivedCache,
  TestApplyAnthropicCacheControlLongLived).
* tests/run_agent/test_anthropic_prompt_cache_policy.py: +9 tests
  (TestSupportsLongLivedAnthropicCache matrix across 8 endpoint classes
  + a fallback-target case).
* tests/agent/test_prompt_caching_live.py: new live E2E (skipif when
  OPENROUTER_API_KEY is unset; runs outside the hermetic suite).
* Targeted suites: 327/327 pass (caching/adapter/policy/builder).
* tests/agent/ + tests/run_agent/: 3992 pass, 17 skip, 1 pre-existing
  flake (test_async_httpx_del_neuter::test_same_key_replaces_stale_loop_entry,
  verified failing on pristine origin/main).

agent/anthropic_adapter.py

@@ -1289,13 +1289,21 @@ def convert_tools_to_anthropic(tools: List[Dict]) -> List[Dict]:
             continue
         if name:
             seen_names.add(name)
-        result.append({
+        anthropic_tool: Dict[str, Any] = {
             "name": name,
             "description": fn.get("description", ""),
             "input_schema": _normalize_tool_input_schema(
                 fn.get("parameters", {"type": "object", "properties": {}})
             ),
-        })
+        }
+        # Forward cache_control marker when present on the OpenAI-format
+        # tool dict (set by ``mark_tools_for_long_lived_cache``). Anthropic's
+        # tools array supports cache_control on the last tool to cache the
+        # entire schema cross-session.
+        cache_control = t.get("cache_control")
+        if isinstance(cache_control, dict):
+            anthropic_tool["cache_control"] = dict(cache_control)
+        result.append(anthropic_tool)
     return result