Add Cerebras platform + generalize StrictOpenAi dialect (provider 10)

The 13th registry row: id "cerebras", CEREBRAS_API_KEY > auth.json
"cerebras" scope, https://api.cerebras.ai/v1 with KIGI_CEREBRAS_BASE_URL
override, Bearer, ChatCompletions, enrichment-backed metadata
(models_dev_id cerebras).

Cerebras' catalog is all chat LLMs (no embedding/tts pollution) and its
/models is minimal (ids only), so restrict_to_enriched=FALSE: keep every
live model, enrich the known ones (context + effort menus low/medium/high),
unknown ones keep the default context. The e2e pins this enrich-without-
restrict path (new — prior enrichment providers all used restrict=true).

Review caught a likely-DOA defect: Cerebras uses strict
additionalProperties:false validation (confirmed 400-rejecting store,
maxTokens, thinking, nested reasoning_content), and stream_options is not
in its schema — so Passthrough (which keeps the stream_options.include_usage
kigi injects on every streaming request) would very likely 400 all
streaming. Generalized ChatCompat::Mistral -> ChatCompat::StrictOpenAi
(serde alias "mistral" keeps pre-rename persisted sessions loading), which
strips stream_options + private fields for any strict OpenAI-compat
validator; both Mistral and Cerebras now map to it. Future strict-validator
candidates (NVIDIA/Azure/Xiaomi/OpenCode) noted for the same check.

reasoning_effort (incl. "none") passes through; /v1/models requires auth
so key validation works; console cloud.cerebras.ai.

crates/codegen/kigi-shell/src/agent/auth_method.rs

@@ -604,7 +604,8 @@ mod tests {
                 "fireworks",
                 "google",
                 "openrouter",
-                "together"
+                "together",
+                "cerebras"
             ]
         );
         assert_eq!(default_id(&built), Some(XAI_API_KEY_METHOD_ID));
@@ -639,7 +640,8 @@ mod tests {
                 "fireworks",
                 "google",
                 "openrouter",
-                "together"
+                "together",
+                "cerebras"
             ]
         );
         assert_eq!(default_id(&built), Some(CACHED_TOKEN_AUTH_METHOD_ID));
@@ -667,7 +669,8 @@ mod tests {
                 "fireworks",
                 "google",
                 "openrouter",
-                "together"
+                "together",
+                "cerebras"
             ]
         );
         assert_eq!(default_id(&built), Some(CACHED_TOKEN_AUTH_METHOD_ID));
@@ -698,7 +701,8 @@ mod tests {
                 "fireworks",
                 "google",
                 "openrouter",
-                "together"
+                "together",
+                "cerebras"
             ]
         );
         assert_eq!(default_id(&built), None);