Use error code instead of string matching for session conflicts

String matching on error.message is fragile - if the message wording
changes, the code silently breaks. Using error.code is type-safe and
reliable.

Author: ghostwriternr

.changeset/improve-session-initialization.md

@@ -2,4 +2,4 @@
 '@cloudflare/sandbox': patch
 ---
 
-Improve session initialization to check storage before creating new session
+Fix session initialization to eliminate noisy error logs during hot reloads

CLAUDE.md

@@ -291,6 +291,8 @@ Turbo handles task orchestration (`turbo.json`) with dependency-aware builds.
 
 **Subject line should stand alone** - don't require reading the body to understand the change. Body is optional and only needed for non-obvious context.
 
+**Focus on the change, not how it was discovered** - never reference "review feedback", "PR comments", or "code review" in commit messages. Describe what the change does and why, not that someone asked for it.
+
 Good examples:
 
 ```

packages/sandbox/src/sandbox.ts

@@ -37,7 +37,8 @@ import {
   CustomDomainRequiredError,
   ErrorCode,
   ProcessExitedBeforeReadyError,
-  ProcessReadyTimeoutError
+  ProcessReadyTimeoutError,
+  SandboxError
 } from './errors';
 import { CodeInterpreter } from './interpreter';
 import { isLocalhostPattern } from './request-handler';
@@ -1014,7 +1015,10 @@ export class Sandbox<Env = unknown> extends Container<Env> implements ISandbox {
       this.logger.debug('Default session initialized', { sessionId });
     } catch (error: unknown) {
       // Session may already exist (e.g., after hot reload or concurrent request)
-      if (error instanceof Error && error.message.includes('already exists')) {
+      if (
+        error instanceof SandboxError &&
+        error.code === ErrorCode.RESOURCE_BUSY
+      ) {
         this.logger.debug(
           'Session exists in container but not in DO state, syncing',
           { sessionId }