fix(cache): enable prompt caching for Qwen on OpenCode/OpenCode-Go/Alibaba (#13528)

Qwen models on OpenCode, OpenCode Go, and direct DashScope accept
Anthropic-style cache_control markers on OpenAI-wire chat completions,
but hermes only injected markers for Claude-named models. Result: zero
cache hits on every turn, full prompt re-billed — a community user
reported burning through their OpenCode Go subscription on Qwen3.6.

Extend _anthropic_prompt_cache_policy to return (True, False) — envelope
layout, not native — for the Alibaba provider family when the model name
contains 'qwen'. Envelope layout places markers on inner content blocks
(matching pi-mono's 'alibaba' cacheControlFormat) and correctly skips
top-level markers on tool-role messages (which OpenCode rejects).

Non-Qwen models on these providers (GLM, Kimi) keep their existing
behaviour — they have automatic server-side caching and don't need
client markers.

Upstream reference: pi-mono #3392 / #3393 documented this contract for
opencode-go Qwen models.

Adds 7 regression tests covering Qwen3.5/3.6/coder on each affected
provider plus negative cases for GLM/Kimi/OpenRouter-Qwen.

tests/run_agent/test_anthropic_prompt_cache_policy.py

@@ -118,6 +118,86 @@ class TestOpenAIWireFormatOnCustomProvider:
         assert agent._anthropic_prompt_cache_policy() == (False, False)
 
 
+class TestQwenAlibabaFamily:
+    """Qwen on OpenCode/OpenCode-Go/Alibaba — needs cache_control even on OpenAI-wire.
+
+    Upstream pi-mono #3392 / #3393 documented that these providers serve
+    zero cache hits without Anthropic-style markers. Regression reported
+    by community user (Qwen3.6 on opencode-go burning through
+    subscription with no cache). Envelope layout, not native, because the
+    wire format is OpenAI chat.completions.
+    """
+
+    def test_qwen_on_opencode_go_caches_with_envelope_layout(self):
+        agent = _make_agent(
+            provider="opencode-go",
+            base_url="https://opencode.ai/v1",
+            api_mode="chat_completions",
+            model="qwen3.6-plus",
+        )
+        should, native = agent._anthropic_prompt_cache_policy()
+        assert should is True, "Qwen on opencode-go must cache"
+        assert native is False, "opencode-go is OpenAI-wire; envelope layout"
+
+    def test_qwen35_plus_on_opencode_go(self):
+        agent = _make_agent(
+            provider="opencode-go",
+            base_url="https://opencode.ai/v1",
+            api_mode="chat_completions",
+            model="qwen3.5-plus",
+        )
+        assert agent._anthropic_prompt_cache_policy() == (True, False)
+
+    def test_qwen_on_opencode_zen_caches(self):
+        agent = _make_agent(
+            provider="opencode",
+            base_url="https://opencode.ai/v1",
+            api_mode="chat_completions",
+            model="qwen3-coder-plus",
+        )
+        assert agent._anthropic_prompt_cache_policy() == (True, False)
+
+    def test_qwen_on_direct_alibaba_caches(self):
+        agent = _make_agent(
+            provider="alibaba",
+            base_url="https://dashscope.aliyuncs.com/compatible-mode/v1",
+            api_mode="chat_completions",
+            model="qwen3-coder",
+        )
+        assert agent._anthropic_prompt_cache_policy() == (True, False)
+
+    def test_non_qwen_on_opencode_go_does_not_cache(self):
+        # GLM / Kimi on opencode-go don't need markers (they have automatic
+        # server-side caching or none at all).
+        agent = _make_agent(
+            provider="opencode-go",
+            base_url="https://opencode.ai/v1",
+            api_mode="chat_completions",
+            model="glm-5",
+        )
+        assert agent._anthropic_prompt_cache_policy() == (False, False)
+
+    def test_kimi_on_opencode_go_does_not_cache(self):
+        agent = _make_agent(
+            provider="opencode-go",
+            base_url="https://opencode.ai/v1",
+            api_mode="chat_completions",
+            model="kimi-k2.5",
+        )
+        assert agent._anthropic_prompt_cache_policy() == (False, False)
+
+    def test_qwen_on_openrouter_not_affected(self):
+        # Qwen via OpenRouter falls through — OpenRouter has its own
+        # upstream caching arrangement for Qwen (provider-dependent).
+        agent = _make_agent(
+            provider="openrouter",
+            base_url="https://openrouter.ai/api/v1",
+            api_mode="chat_completions",
+            model="qwen/qwen3-coder",
+        )
+        assert agent._anthropic_prompt_cache_policy() == (False, False)
+
+
 class TestExplicitOverrides:
     """Policy accepts keyword overrides for switch_model / fallback activation."""