Add Mistral platform + Mistral dialect + array-content handling (provider 5)

The 8th registry row: id "mistral", MISTRAL_API_KEY > auth.json
"mistral" scope, https://api.mistral.ai/v1 with KIGI_MISTRAL_BASE_URL
override, enrichment-backed metadata with the tool-calling listing
restriction (embed/moderation/OCR noise).

Mistral is NOT a pure-pattern provider — an adversarial review found two
doc-confirmed blockers that no test exercises (no e2e covers a chat POST),
so the gate-green registry row alone would have shipped it DOA. A research
workflow pinned the exact wire shapes against the mistralai/client-python
SDK source (adversarially verified), then both were fixed:

1. stream_options 422: Mistral's strict Pydantic validator rejects the
   stream_options.include_usage field kigi injects on every streaming
   request (the SDK's request model has no such field). New
   ChatCompat::Mistral dialect strips it (plus the kigi-private message
   fields, like Passthrough). Streaming usage falls back to token
   estimation.
2. Reasoning content arrays: Mistral reasoning models return content as
   Union[str, List[ContentChunk]] on both streaming and non-streaming,
   which the flat Option<String> path could not decode -> aborted turn.
   A UNIVERSAL lenient deserializer (#[serde(from = "Raw..")] on
   ChatResponseMessage + ChatChunkDelta) accepts string-or-array, routing
   {type:text} chunks to the answer and the nested text of {type:thinking}
   chunks to reasoning_content, tolerant of the OPEN chunk union (unknown
   types ignored, never fatal). String content stays byte-identical for
   every other provider (kimi/deepseek/groq/BYOK).

Review refuted all seven attack lines (no regression, no crash, exhaustive)
and flagged one coverage gap, now closed: a stream-consumer integration
test drives a full thinking -> transition -> answer chunk sequence and
proves it yields the same reasoning-sibling + assistant-answer result as
the reasoning_content string path.

Also folds a verified quirk matrix for all 23 remaining API providers into
providers-plan.md, tiered by real difficulty (self-enriching OpenRouter/
Vercel; bare-array Together listing; Messages-dialect MiniMax reusing the
Anthropic machinery; non-Bearer Azure/Bedrock; router wildcards; the OAuth
block).

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

@@ -595,7 +595,8 @@ mod tests {
                 "openai",
                 "anthropic",
                 "deepseek",
-                "groq"
+                "groq",
+                "mistral"
             ]
         );
         assert_eq!(default_id(&built), Some(XAI_API_KEY_METHOD_ID));
@@ -625,7 +626,8 @@ mod tests {
                 "openai",
                 "anthropic",
                 "deepseek",
-                "groq"
+                "groq",
+                "mistral"
             ]
         );
         assert_eq!(default_id(&built), Some(CACHED_TOKEN_AUTH_METHOD_ID));
@@ -648,7 +650,8 @@ mod tests {
                 "openai",
                 "anthropic",
                 "deepseek",
-                "groq"
+                "groq",
+                "mistral"
             ]
         );
         assert_eq!(default_id(&built), Some(CACHED_TOKEN_AUTH_METHOD_ID));
@@ -674,7 +677,8 @@ mod tests {
                 "openai",
                 "anthropic",
                 "deepseek",
-                "groq"
+                "groq",
+                "mistral"
             ]
         );
         assert_eq!(default_id(&built), None);