Add DeepSeek platform + ChatCompletions dialect system (provider 3)

The 6th registry row: id "deepseek", DEEPSEEK_API_KEY > auth.json
"deepseek" scope, base https://api.deepseek.com (chat rides
{base}/chat/completions per official docs) with KIGI_DEEPSEEK_BASE_URL
override, enrichment-backed metadata (1M context, 384k output cap,
high/max effort menu).

Structural fix the cycle exposed: kigi's Kimi-specific body adaptation
ran UNCONDITIONALLY on every ChatCompletions request. New ChatCompat
dialect, declared per platform row and threaded through SamplerConfig,
ClientDefaults, and the session-persisted SamplingConfig (serde-default
Kimi keeps restored pre-field sessions and BYOK endpoints byte-identical;
production persist seams copy it; subagents inherit it):
- Kimi: full legacy pipeline (dispatch ≡ legacy pinned)
- DeepSeek: thinking:{type, reasoning_effort} per api-docs.deepseek.com
  (server maps low/medium→high, xhigh→max itself; none disables; absent
  leaves the server default)
- Passthrough: OpenAI-style reasoning_effort scalar untouched (unblocks
  Groq and the rest of the OpenAI-compatible list)

Review-confirmed release blocker fixed: kigi replays Kimi's
reasoning_content (and its private model_id) on input assistant
messages — Kimi consumes these, but DeepSeek documents input
reasoning_content as prefix-mode-only (historically a 400) and other
providers don't know either field. The DeepSeek and Passthrough arms now
strip both; Kimi's own pipeline is untouched. Pinned on both message
shapes.

crates/codegen/kigi-sampler/src/client.rs

@@ -270,6 +270,7 @@ struct ClientDefaults {
     top_p: Option<f32>,
     api_backend: ApiBackend,
     auth_scheme: AuthScheme,
+    chat_compat: kigi_sampling_types::ChatCompat,
     stream_tool_calls: bool,
     doom_loop_recovery: Option<kigi_sampling_types::DoomLoopRecoveryPolicy>,
 }
@@ -456,6 +457,7 @@ impl SamplingClient {
             top_p: config.top_p,
             api_backend: config.api_backend,
             auth_scheme: config.auth_scheme,
+            chat_compat: config.chat_compat,
             stream_tool_calls: config.stream_tool_calls,
             doom_loop_recovery: config.doom_loop_recovery,
         };
@@ -775,7 +777,10 @@ impl SamplingClient {
             tracing::error!("Failed to serialize chat/completions request: {}", e);
             SamplingError::Serialization(e)
         })?;
-        crate::kimi_compat::adapt_chat_completions_body(&mut request_body);
+        crate::kimi_compat::adapt_chat_completions_body_for(
+            self.defaults.chat_compat,
+            &mut request_body,
+        );
 
         let http_request = self
             .post(self.endpoint("chat/completions"))
@@ -829,7 +834,10 @@ impl SamplingClient {
             tracing::error!("Failed to serialize chat/completions request: {}", e);
             SamplingError::Serialization(e)
         })?;
-        crate::kimi_compat::adapt_chat_completions_body(&mut request_body);
+        crate::kimi_compat::adapt_chat_completions_body_for(
+            self.defaults.chat_compat,
+            &mut request_body,
+        );
 
         let http_request = self
             .post(self.endpoint("chat/completions"))
@@ -1880,6 +1888,7 @@ mod tests {
             top_p: None,
             api_backend: ApiBackend::ChatCompletions,
             auth_scheme: AuthScheme::Bearer,
+            chat_compat: Default::default(),
             extra_headers: IndexMap::new(),
             context_window: 8192,
             force_http1: false,
@@ -2076,6 +2085,7 @@ mod tests {
             api_key: Some("bearer-key-abc123".to_string()),
             api_backend: ApiBackend::Messages,
             auth_scheme: AuthScheme::Bearer,
+            chat_compat: Default::default(),
             ..minimal_config()
         };
         let client = SamplingClient::new(cfg).expect("client should build");
@@ -2242,6 +2252,7 @@ mod tests {
             api_key: Some("stale-bearer".to_string()),
             api_backend: ApiBackend::Messages,
             auth_scheme: AuthScheme::Bearer,
+            chat_compat: Default::default(),
             bearer_resolver: Some(std::sync::Arc::new(StaticBearerResolver("fresh-bearer"))),
             ..minimal_config()
         };
@@ -2270,6 +2281,7 @@ mod tests {
             api_key: Some("stale-bearer".to_string()),
             api_backend: ApiBackend::Responses,
             auth_scheme: AuthScheme::Bearer,
+            chat_compat: Default::default(),
             bearer_resolver: Some(std::sync::Arc::new(StaticBearerResolver("fresh-bearer"))),
             ..minimal_config()
         };