- Remove tests for `this` serialization in step functions since the SWC
  plugin forbids `this` usage in steps (ChainableService and
  thisSerializationWorkflow tests disabled with comment)
- Use Symbol.for() directly in 99_e2e.ts instead of importing from
  'workflow' to avoid pulling in server-side dependencies (node:async_hooks)
  when the file is bundled for the client

Both Next.js turbopack and webpack builds now pass successfully.

This reverts commit 0fc1fae.

Use Symbol.for() directly in 99_e2e.ts instead of importing from 'workflow'
to avoid pulling in server-side dependencies (node:async_hooks) when the
file is bundled for the client in the workbench Next.js apps.

This is an internal function only used by SWC-generated code, which imports
it from workflow/internal/class-serialization instead.

The SWC plugin specifically looks for the `Symbol.for('workflow-serialize')`
pattern in computed property names. Using a variable reference like
`[WORKFLOW_SERIALIZE]` doesn't work because the plugin does AST pattern
matching, not runtime evaluation.

… symbols

The SWC plugin now recognizes custom serialization classes using any of these patterns:

1. Direct Symbol.for() in class definition:
   static [Symbol.for('workflow-serialize')](instance) { ... }

2. Imported symbols from 'workflow' package:
   import { WORKFLOW_SERIALIZE } from 'workflow';
   static [WORKFLOW_SERIALIZE](instance) { ... }

3. Local const defined with Symbol.for():
   const MY_SYM = Symbol.for('workflow-serialize');
   static [MY_SYM](instance) { ... }

4. Re-exported symbols from other modules

Added test fixtures for imported and local const patterns.

Sort classes_needing_serialization before iterating to ensure
deterministic output ordering in test fixtures.

The serializer needs the classId property on the class to serialize
instances (e.g., step return values). Previously this was only set
in workflow mode by the SWC plugin, but step mode also needs it for
serializing return values.

…d workflow modes

- Move class registry to globalThis for isomorphic access in both contexts
- Add WORKFLOW_CLASS_REGISTRY symbol to symbols.ts
- Refactor class-serialization.ts to use globalThis-based registry that is
  lazily initialized (works in both step mode and VM workflow mode)
- Update SWC plugin workflow mode to import and call registerSerializationClass()
  instead of manual classId assignment and globalThis[].set() calls
- Add fallback in serialization.ts reviver to check VM's globalThis registry
- Update test fixtures for the new import/call pattern

This simplifies the code by using the same registerSerializationClass() function
in both step and workflow modes, rather than generating different code for each.