fix(kiro): improve direct session cache reuse

Reshape Kiro direct requests so resumed client sessions reuse Kiro's
cache-affinity fields instead of starting unrelated CodeWhisperer
conversations.

- keep conversationState.conversationId stable when the client sends an
  explicit session id (x-session-id, session_id, conversation_id, Claude
  Code session metadata)
- add a stable conversationState.agentContinuationId per Kiro session
- send conversationState.agentTaskType: "vibe" and agentMode: "vibe",
  matching the normal Kiro CLI/KAS chat path
- move Kiro thinking instructions into Kiro-compatible systemPrompt /
  additionalModelRequestFields instead of generic top-level thinking
- keep volatile timestamp context out of the top-level systemPrompt; it
  remains only in user content fallback
- suppress additionalModelRequestFields for legacy 4.5-era Claude/Kiro
  models that reject it, while defaulting future Claude/Kiro model ids
  to supported
- preserve Kiro meteringEvent credit usage internally for accounting
  without leaking provider-specific fields into OpenAI-compatible usage
- prevent unrelated headerless Kiro requests from sharing one
  connection-wide continuation
- cap/evict continuation sessions so long-running processes do not grow
  the continuation map unbounded
- treat generated headerless Kiro sessions as one-shot so they do not
  evict real explicit-session continuations
- keep credit-only Kiro metering valid for internal persistence when
  token metrics are unavailable

open-sse/config/kiroConstants.js

@@ -131,6 +131,50 @@ export function resolveKiroThinkingBudget(body, headers, model) {
   return null;
 }
 
+export function extractKiroEffortLevel(body) {
+  const effort =
+    body?.output_config?.effort ??
+    body?.reasoning_effort ??
+    (typeof body?.reasoning === "object" ? body.reasoning?.effort : null);
+  if (typeof effort !== "string") return null;
+  const normalized = effort.toLowerCase();
+  if (normalized === "none" || normalized === "off" || normalized === "disabled") return null;
+  if (normalized === "xhigh" || normalized === "max") return "high";
+  if (["low", "medium", "high"].includes(normalized)) return normalized;
+  return null;
+}
+
+export function buildKiroAdditionalModelRequestFields(body) {
+  const effort = extractKiroEffortLevel(body);
+  if (!effort) return undefined;
+  // Mirrors Kiro CLI/KAS buildEffortRequestFields("output_config").
+  return {
+    thinking: { type: "adaptive", display: "summarized" },
+    output_config: { effort },
+  };
+}
+
+export function supportsKiroAdditionalModelRequestFields(model) {
+  if (typeof model !== "string") return false;
+  const normalized = model.toLowerCase().replace(/-/g, ".");
+  if (!normalized.includes("claude")) return false;
+  const match = normalized.match(/(?:^|[/.])claude(?:[/.][a-z]+)*[/.](\d+)(?:[/.](\d+))?(?:[/.]|$)/);
+  if (!match) return false;
+  const [, majorText, minorText] = match;
+  const major = Number(majorText);
+  const minor = minorText === undefined ? null : Number(minorText);
+  const dateSuffixMinor = minor !== null && minor >= 1000;
+  // Kiro rejected additionalModelRequestFields on legacy 4.5 models in live smoke.
+  // Default future Claude/Kiro models to supported so new model releases do not
+  // need a code allowlist update.
+  return !(major < 4 || (major === 4 && (minor === null || minor <= 5 || dateSuffixMinor)));
+}
+
+export function buildKiroAdditionalModelRequestFieldsForModel(body, model) {
+  if (!supportsKiroAdditionalModelRequestFields(model)) return undefined;
+  return buildKiroAdditionalModelRequestFields(body);
+}
+
 /**
  * Detect whether an inbound request is asking for reasoning / thinking output.
  * Thin wrapper over resolveKiroThinkingBudget (single source of truth).