feat: add type guard helpers for ContentBlock variants

[ac12644]

Summary

• Adds 12 type guard functions (isTextBlock, isToolUseBlock, isThinkingBlock, etc.) that narrow ContentBlock to its specific variant
• Exported from the package root for ergonomic usage
• Includes compile-time exhaustiveness test that fails if a new ContentBlock variant is added without a corresponding guard

Motivation

Addresses #432 — the most upvoted open issue (15+ thumbs up). Since v0.22.0, ContentBlock is a 12-member union, so msg.content[0].text requires type narrowing. Users currently resort to as casts or // @ts-ignore.

With this change:

import { isTextBlock } from '@anthropic-ai/sdk';

const message = await client.messages.create({ ... });
const textBlocks = message.content.filter(isTextBlock);
console.log(textBlocks[0].text); // fully typed, no casts

Design decisions

• Type guards over overloads: Overloading create() based on tools presence would be type-unsound (thinking, server tools, etc. can produce non-text blocks even without tools). Type guards are correct and composable.
• src/lib/ placement: Hand-written helpers that survive Stainless code generation, following the existing pattern (parser.ts, MessageStream.ts).
• Exhaustiveness test: Uses compile-time conditional types (same pattern as TracksToolInput.test.ts) so adding a new ContentBlock variant without a guard is a build error.

Test plan

• yarn lint passes (types + eslint + prettier)
• 18 Jest tests pass (runtime behavior + type narrowing for all 12 guards)
• Compile-time exhaustiveness check covers all 12 variants
• Existing tests unaffected (TracksToolInput, index.test, parser.test)
• Full build passes (CJS + ESM)
• Verified imports work from built dist (require and import)

Closes #432