fix(chat): BYOK dialect defaults to Passthrough; dedicated Mistral dialect

C1 (decision): custom/BYOK ChatCompletions entries defaulted to the Kimi
dialect, leaking Kimi-specific body mutations (thinking:{…} control,
replayed reasoning_content, schema rewrites) to arbitrary third-party
OpenAI-compatible servers. The default is now Passthrough (vanilla
OpenAI semantics), with ONE exception mirroring Pi's base-url quirk
sniffing: entries pointed at the house/Kimi coding endpoint keep the
Kimi dialect. Registry platforms are unaffected (all declare
explicitly).

C2 (Pi mistral-conversations normalizer): Mistral requires tool-call ids
of EXACTLY nine [a-zA-Z0-9] chars; even same-session synthesized UUIDs
violate it. New ChatCompat::Mistral = StrictOpenAi behavior + the
normalizer — strip non-alphanumerics, keep exact-9 ids, else FNV-1a →
base36 (build-stable, deterministic across requests for prefix-cache
stability) with collision retry; ONE map covers tool_calls[].id and
tool_call_id so pairing survives. The mistral registry row and the
persisted 'mistral' serde value both resolve to it (pre-rename Mistral
sessions gain the contract automatically); Cerebras/NVIDIA stay on
StrictOpenAi untouched.

Verified: models+sampling-types+sampler+chat-state+shell all green,
clippy clean.

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

@@ -1841,8 +1841,9 @@ mod tests {
         let cfg = crate::agent::config::sampling_config_for_model(&model_entry, creds, None);
         assert_eq!(
             cfg.chat_compat,
-            kigi_sampling_types::ChatCompat::StrictOpenAi,
-            "mistral entries use the StrictOpenAi dialect (stream_options strip)"
+            kigi_sampling_types::ChatCompat::Mistral,
+            "mistral entries use the Mistral dialect (StrictOpenAi behavior \
+             plus the exactly-nine-alphanumeric tool-call id contract)"
         );
     }