feat: add library-based ts-to-zod schema generation

Add auto-generated Zod schemas from spec.types.ts using ts-to-zod as a library,
with post-processing for SDK compatibility.

## What's New

- `src/generated/spec.schemas.ts` - 145 auto-generated Zod schemas
- `src/generated/index.ts` - Public API with explicit exports
- `test/generated/spec.schemas.compare.test.ts` - 97 tests verifying equivalence
- `scripts/generate-schemas.ts` - Library-based generator with post-processing

## Post-Processing

The generator applies several transformations for SDK compatibility:

1. **Zod v4 import** - `"zod"` → `"zod/v4"`
2. **Index signatures** - `z.record().and(z.object())` → `z.looseObject()`
3. **typeof expressions** - `jsonrpc: z.any()` → `z.literal("2.0")`
4. **SDK hierarchy** - Notifications/Requests extend `NotificationSchema`/`RequestSchema`
   instead of `JSONRPCNotificationSchema`/`JSONRPCRequestSchema`

## Usage

```typescript
// Generated schemas (drop-in compatible with types.ts)
import { ProgressTokenSchema } from '@modelcontextprotocol/sdk/generated';

// Or continue using production schemas
import { ProgressTokenSchema } from '@modelcontextprotocol/sdk/types.js';
```

## Design Decisions

**Why types.ts is unchanged:**
- Some manual schemas have refinements (`.int()`) not in generated schemas
- 10 SDK-specific schemas not in spec (CompatibilityCallToolResultSchema, etc.)
- Type guards (isJSONRPCRequest, etc.) remain manual
- Full refactor to thin re-export layer is future work

**Explicit exports in generated/index.ts:**
- Each schema and type listed by name for legibility
- Grouped by category (primitives, resources, tools, etc.)
- No `export *` to avoid accidental internal exports

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

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

Author: ochafik