fix(minimax): correct context lengths, model catalog, thinking guard, aux model, and config base_url

Cherry-picked from PR #6046 by kshitijk4poor with dead code stripped.

- Context lengths: 204800 → 1M (M1) / 1048576 (M2.5/M2.7) per official docs
- Model catalog: add M1 family, remove deprecated M2.1 and highspeed variants
- Thinking guard: skip extended thinking for MiniMax (Anthropic-compat endpoint)
- Aux model: MiniMax-M2.7-highspeed → MiniMax-M2.7 (same model, half price)
- Config base_url: honour model.base_url for API-key providers (fixes China users)
- Stripped unused get_minimax_max_output() / _MINIMAX_MAX_OUTPUT (no consumer)

Fixes #5777, #4082, #6039. Closes #3895.

agent/model_metadata.py

@@ -113,8 +113,15 @@ DEFAULT_CONTEXT_LENGTHS = {
     "llama": 131072,
     # Qwen
     "qwen": 131072,
-    # MiniMax
-    "minimax": 204800,
+    # MiniMax (lowercase — lookup lowercases model names at line 973)
+    "minimax-m1-256k": 1000000,
+    "minimax-m1-128k": 1000000,
+    "minimax-m1-80k": 1000000,
+    "minimax-m1-40k": 1000000,
+    "minimax-m1": 1000000,
+    "minimax-m2.5": 1048576,
+    "minimax-m2.7": 1048576,
+    "minimax": 1048576,
     # GLM
     "glm": 202752,
     # Kimi
@@ -127,7 +134,7 @@ DEFAULT_CONTEXT_LENGTHS = {
     "deepseek-ai/DeepSeek-V3.2": 65536,
     "moonshotai/Kimi-K2.5": 262144,
     "moonshotai/Kimi-K2-Thinking": 262144,
-    "MiniMaxAI/MiniMax-M2.5": 204800,
+    "minimaxai/minimax-m2.5": 1048576,
     "XiaomiMiMo/MiMo-V2-Flash": 32768,
     "mimo-v2-pro": 1048576,
     "mimo-v2-omni": 1048576,