feat(catalog): add zai/glm-5.1, zai/glm-4.7-flash, openrouter/z-ai/glm-5.1

Three new pricing entries for Z.AI's GLM family, all sourced from official
upstream documentation:

* `zai/glm-5.1` (Z.AI direct, provider=zai)
  - input $1.40/1M, cached input $0.26/1M, output $4.40/1M
  - context 200K in / 128K out (matching sibling zai/glm-5 / zai/glm-4.7)
  - source: https://docs.z.ai/guides/overview/pricing

* `zai/glm-4.7-flash` (Z.AI direct, provider=zai)
  - all four price columns published as Free on the Z.AI pricing page
    (input / cached input / cache storage / output)
  - context 200K in / 128K out
  - supports_prompt_caching: true and supports_reasoning: true to match
    sibling zai/glm-4.7 conventions; the Z.AI pricing page lists a cached-
    input column for this model (free), so the model is caching-eligible
  - source: https://docs.z.ai/guides/overview/pricing

* `openrouter/z-ai/glm-5.1` (OpenRouter route, provider=openrouter)
  - input $1.05/1M, cached input $0.525/1M, output $3.50/1M
  - max_input_tokens 202752, max_output_tokens 65535
    (matches OpenRouter's `top_provider.max_completion_tokens` for this
    model; intentionally lower than Z.AI direct's 128K since this is the
    OR endpoint's actual ceiling)
  - source: https://openrouter.ai/z-ai/glm-5.1

All three entries follow the conventions of surrounding entries
(zai/glm-5 and zai/glm-4.7 for the zai/* namespace, openrouter/z-ai/glm-5
for the openrouter/z-ai/* namespace).

Why: direct Z.AI users and openrouter users on these models currently get
response_cost = 0.0 in spend logs because the cost calculator falls back
to the static catalog and finds nothing for these newer models. With these
entries, cost calc picks them up by router_model_id, so spend tracking and
budget enforcement work without per-deployment input_cost_per_token /
output_cost_per_token overrides on every install.

Verified empirically against:
- https://docs.z.ai/guides/overview/pricing (2026-05-07) for the two
  zai/* entries.
- https://openrouter.ai/api/v1/models (2026-05-07) for the
  openrouter/z-ai/glm-5.1 entry. The 65535 max_output_tokens value comes
  directly from `top_provider.max_completion_tokens` in the JSON response.

Test plan:
- `python3 -c "import json; json.load(open('model_prices_and_context_window.json'))"` — JSON parses.
- All three keys / values inspected against neighbour conventions; no
  duplicate fields, no removed fields.
- No code-path changes, so no test additions are needed in tests/litellm/.

model_prices_and_context_window.json

@@ -27403,6 +27403,22 @@
         "supports_reasoning": true,
         "supports_tool_choice": true
     },
+    "openrouter/z-ai/glm-5.1": {
+        "input_cost_per_token": 1.05e-06,
+        "output_cost_per_token": 3.5e-06,
+        "cache_read_input_token_cost": 5.25e-07,
+        "cache_creation_input_token_cost": 0.0,
+        "litellm_provider": "openrouter",
+        "max_input_tokens": 202752,
+        "max_output_tokens": 65535,
+        "max_tokens": 65535,
+        "mode": "chat",
+        "source": "https://openrouter.ai/z-ai/glm-5.1",
+        "supports_function_calling": true,
+        "supports_prompt_caching": true,
+        "supports_reasoning": true,
+        "supports_tool_choice": true
+    },
     "openrouter/minimax/minimax-m2.1": {
         "input_cost_per_token": 2.7e-07,
         "output_cost_per_token": 1.2e-06,
@@ -35130,6 +35146,21 @@
         "supports_tool_choice": true,
         "source": "https://docs.z.ai/guides/overview/pricing"
     },
+    "zai/glm-5.1": {
+        "cache_creation_input_token_cost": 0,
+        "cache_read_input_token_cost": 2.6e-07,
+        "input_cost_per_token": 1.4e-06,
+        "output_cost_per_token": 4.4e-06,
+        "litellm_provider": "zai",
+        "max_input_tokens": 200000,
+        "max_output_tokens": 128000,
+        "mode": "chat",
+        "supports_function_calling": true,
+        "supports_prompt_caching": true,
+        "supports_reasoning": true,
+        "supports_tool_choice": true,
+        "source": "https://docs.z.ai/guides/overview/pricing"
+    },
     "zai/glm-5-code": {
         "cache_creation_input_token_cost": 0,
         "cache_read_input_token_cost": 3e-07,
@@ -35160,6 +35191,21 @@
         "supports_tool_choice": true,
         "source": "https://docs.z.ai/guides/overview/pricing"
     },
+    "zai/glm-4.7-flash": {
+        "cache_creation_input_token_cost": 0,
+        "cache_read_input_token_cost": 0,
+        "input_cost_per_token": 0,
+        "output_cost_per_token": 0,
+        "litellm_provider": "zai",
+        "max_input_tokens": 200000,
+        "max_output_tokens": 128000,
+        "mode": "chat",
+        "supports_function_calling": true,
+        "supports_prompt_caching": true,
+        "supports_reasoning": true,
+        "supports_tool_choice": true,
+        "source": "https://docs.z.ai/guides/overview/pricing"
+    },
     "zai/glm-4.6": {
         "cache_creation_input_token_cost": 0,
         "cache_read_input_token_cost": 1.1e-07,