fix(metadata): qwen3.6-plus has a 1M context window (#27008)

qwen3.6-plus did not have an explicit entry in DEFAULT_CONTEXT_LENGTHS,
so the longest-substring fallback matched the generic 'qwen': 131072
catch-all. That dropped the effective context limit from 1,048,576
tokens to 131,072, prematurely lowered the compression threshold, and
produced misleading warnings about main/compression context mismatch
in long sessions.

Add an explicit 'qwen3.6-plus': 1048576 entry before the catch-all and
cover it with a regression test (bare, qwen/, and dashscope/ prefixes).

Note: PR #6599 also mentions touching model_metadata.py but the actual
diff only edits hermes_cli/models.py, so this fix is independent and
not duplicated by that PR.

Closes #27008

agent/model_metadata.py

@@ -194,6 +194,7 @@ DEFAULT_CONTEXT_LENGTHS = {
     "llama": 131072,
     # Qwen — specific model families before the catch-all.
     # Official docs: https://help.aliyun.com/zh/model-studio/developer-reference/
+    "qwen3.6-plus": 1048576,      # 1M context (DashScope/Alibaba & OpenRouter)
     "qwen3-coder-plus": 1000000,  # 1M context
     "qwen3-coder": 262144,        # 256K context
     "qwen": 131072,