refactor: centralize constants, add error classes, and DRY test utilities

- Create src/errors.js with custom error classes (RateLimitError, AuthError, ApiError, etc.)
- Create src/utils/helpers.js with shared utilities (formatDuration, sleep)
- Create tests/helpers/http-client.cjs with shared test utilities (~250 lines deduped)
- Centralize OAuth config and other constants in src/constants.js
- Add JSDoc types to all major exported functions
- Refactor all test files to use shared http-client utilities
- Update CLAUDE.md with new architecture documentation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

tests/test-interleaved-thinking.cjs

@@ -9,127 +9,10 @@
  * This simulates complex Claude Code scenarios where the model
  * thinks multiple times during a single turn.
  */
-const http = require('http');
-
-const BASE_URL = 'localhost';
-const PORT = 8080;
-
-function streamRequest(body) {
-    return new Promise((resolve, reject) => {
-        const data = JSON.stringify(body);
-        const req = http.request({
-            host: BASE_URL,
-            port: PORT,
-            path: '/v1/messages',
-            method: 'POST',
-            headers: {
-                'Content-Type': 'application/json',
-                'x-api-key': 'test',
-                'anthropic-version': '2023-06-01',
-                'anthropic-beta': 'interleaved-thinking-2025-05-14',
-                'Content-Length': Buffer.byteLength(data)
-            }
-        }, res => {
-            const events = [];
-            let fullData = '';
-
-            res.on('data', chunk => {
-                fullData += chunk.toString();
-            });
-
-            res.on('end', () => {
-                const parts = fullData.split('\n\n').filter(e => e.trim());
-                for (const part of parts) {
-                    const lines = part.split('\n');
-                    const eventLine = lines.find(l => l.startsWith('event:'));
-                    const dataLine = lines.find(l => l.startsWith('data:'));
-                    if (eventLine && dataLine) {
-                        try {
-                            const eventType = eventLine.replace('event:', '').trim();
-                            const eventData = JSON.parse(dataLine.replace('data:', '').trim());
-                            events.push({ type: eventType, data: eventData });
-                        } catch (e) { }
-                    }
-                }
-
-                const content = [];
-                let currentBlock = null;
-
-                for (const event of events) {
-                    if (event.type === 'content_block_start') {
-                        currentBlock = { ...event.data.content_block };
-                        if (currentBlock.type === 'thinking') {
-                            currentBlock.thinking = '';
-                            currentBlock.signature = '';
-                        }
-                        if (currentBlock.type === 'text') currentBlock.text = '';
-                    } else if (event.type === 'content_block_delta') {
-                        const delta = event.data.delta;
-                        if (delta.type === 'thinking_delta' && currentBlock) {
-                            currentBlock.thinking += delta.thinking || '';
-                        }
-                        if (delta.type === 'signature_delta' && currentBlock) {
-                            currentBlock.signature += delta.signature || '';
-                        }
-                        if (delta.type === 'text_delta' && currentBlock) {
-                            currentBlock.text += delta.text || '';
-                        }
-                        if (delta.type === 'input_json_delta' && currentBlock) {
-                            currentBlock.partial_json = (currentBlock.partial_json || '') + delta.partial_json;
-                        }
-                    } else if (event.type === 'content_block_stop') {
-                        if (currentBlock?.type === 'tool_use' && currentBlock.partial_json) {
-                            try { currentBlock.input = JSON.parse(currentBlock.partial_json); } catch (e) { }
-                            delete currentBlock.partial_json;
-                        }
-                        if (currentBlock) content.push(currentBlock);
-                        currentBlock = null;
-                    }
-                }
-
-                const errorEvent = events.find(e => e.type === 'error');
-                if (errorEvent) {
-                    resolve({ content, events, error: errorEvent.data.error, statusCode: res.statusCode });
-                } else {
-                    resolve({ content, events, statusCode: res.statusCode });
-                }
-            });
-        });
-        req.on('error', reject);
-        req.write(data);
-        req.end();
-    });
-}
+const { streamRequest, commonTools } = require('./helpers/http-client.cjs');
 
 // Multiple tools to encourage interleaved thinking
-const tools = [{
-    name: 'read_file',
-    description: 'Read a file',
-    input_schema: {
-        type: 'object',
-        properties: { path: { type: 'string' } },
-        required: ['path']
-    }
-}, {
-    name: 'write_file',
-    description: 'Write to a file',
-    input_schema: {
-        type: 'object',
-        properties: {
-            path: { type: 'string' },
-            content: { type: 'string' }
-        },
-        required: ['path', 'content']
-    }
-}, {
-    name: 'run_tests',
-    description: 'Run test suite',
-    input_schema: {
-        type: 'object',
-        properties: { pattern: { type: 'string' } },
-        required: ['pattern']
-    }
-}];
+const tools = [commonTools.readFile, commonTools.writeFile, commonTools.runTests];
 
 async function runTests() {
     console.log('='.repeat(60));