fix(gemini): add conversation history truncation to prevent O(N²) token cost growth

GeminiAgent sends the full conversation history with every API call,
causing quadratic token growth per session. A 100-observation session
sends ~30M cumulative input tokens. This ports the proven truncateHistory()
sliding window from OpenRouterAgent to GeminiAgent.

- Add CLAUDE_MEM_GEMINI_MAX_CONTEXT_MESSAGES (default: 20) and
  CLAUDE_MEM_GEMINI_MAX_TOKENS (default: 100000) settings
- Add truncateHistory() to GeminiAgent using shared estimateTokens()
- Always preserve at least the newest message to avoid empty API requests
- Add settings validation in SettingsRoutes (1-100 messages, 1K-1M tokens)
- Add regression tests for truncation and oversized single-prompt edge case

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/services/worker/http/routes/SettingsRoutes.ts

@@ -94,6 +94,8 @@ export class SettingsRoutes extends BaseRouteHandler {
       'CLAUDE_MEM_GEMINI_API_KEY',
       'CLAUDE_MEM_GEMINI_MODEL',
       'CLAUDE_MEM_GEMINI_RATE_LIMITING_ENABLED',
+      'CLAUDE_MEM_GEMINI_MAX_CONTEXT_MESSAGES',
+      'CLAUDE_MEM_GEMINI_MAX_TOKENS',
       // OpenRouter Configuration
       'CLAUDE_MEM_OPENROUTER_API_KEY',
       'CLAUDE_MEM_OPENROUTER_MODEL',
@@ -248,6 +250,22 @@ export class SettingsRoutes extends BaseRouteHandler {
       }
     }
 
+    // Validate CLAUDE_MEM_GEMINI_MAX_CONTEXT_MESSAGES
+    if (settings.CLAUDE_MEM_GEMINI_MAX_CONTEXT_MESSAGES) {
+      const count = parseInt(settings.CLAUDE_MEM_GEMINI_MAX_CONTEXT_MESSAGES, 10);
+      if (isNaN(count) || count < 1 || count > 100) {
+        return { valid: false, error: 'CLAUDE_MEM_GEMINI_MAX_CONTEXT_MESSAGES must be between 1 and 100' };
+      }
+    }
+
+    // Validate CLAUDE_MEM_GEMINI_MAX_TOKENS
+    if (settings.CLAUDE_MEM_GEMINI_MAX_TOKENS) {
+      const tokens = parseInt(settings.CLAUDE_MEM_GEMINI_MAX_TOKENS, 10);
+      if (isNaN(tokens) || tokens < 1000 || tokens > 1000000) {
+        return { valid: false, error: 'CLAUDE_MEM_GEMINI_MAX_TOKENS must be between 1000 and 1000000' };
+      }
+    }
+
     // Validate CLAUDE_MEM_CONTEXT_OBSERVATIONS
     if (settings.CLAUDE_MEM_CONTEXT_OBSERVATIONS) {
       const obsCount = parseInt(settings.CLAUDE_MEM_CONTEXT_OBSERVATIONS, 10);