fix(agent): comprehensive DeepSeek V4 support — context windows, thinking mode, reasoning replay

Unifies approaches from PRs #14952, #14958, #15325, #15228, #15354
into a single cohesive implementation:

- Add 1M context window entries for V4 models (deepseek-v4-pro,
  deepseek-v4-flash, deepseek-chat, deepseek-reasoner)
- Plumb thinking.type toggle and reasoning_effort mapping for
  native DeepSeek API (only "high" and "max" are valid)
- Strip incompatible sampling params when thinking is enabled
- Inject reasoning_content="" on all assistant messages for
  DeepSeek replay (scoped to api.deepseek.com and OpenRouter)
- Fix _extract_reasoning isinstance checks for empty strings
- Preserve empty-string reasoning_content in normalize_response
- Add _copy_reasoning_content_for_api call in _handle_max_iterations

Fixes #15353. Supersedes #14952, #14958, #15325, #15228, #15354.

agent/model_metadata.py

@@ -162,8 +162,12 @@ DEFAULT_CONTEXT_LENGTHS = {
     "gemma-4-31b": 256000,
     "gemma-3": 131072,
     "gemma": 8192,  # fallback for older gemma models
-    # DeepSeek
-    "deepseek": 128000,
+    # DeepSeek — V4 family supports 1M context (api.deepseek.com docs)
+    "deepseek-v4-pro": 1000000,
+    "deepseek-v4-flash": 1000000,
+    "deepseek-chat": 1000000,
+    "deepseek-reasoner": 1000000,
+    "deepseek": 128000,  # fallback for older/unrecognised DeepSeek models
     # Meta
     "llama": 131072,
     # Qwen — specific model families before the catch-all.