Fix sparse Fern shard aggregate barrel generation

[createsomethingtoday]

Problem

Sparse Fern TypeScript shards can omit api/resources or other aggregate directories. The merge script writes aggregate barrel files into those directories without creating their parents, so otherwise successful generation ends with ENOENT for sdk/api/resources/index.ts. The SDK map emitter also writes an extra blank line at the end of sdk-operation-types.ts, making a raw generated artifact fail git diff --check.

Change

• Create the parent directory before writing the resource barrel and each remaining aggregate barrel.
• Add regression fixtures covering missing resource/core directories and a shard with no aggregate directories.
• Emit exactly one terminal newline in sdk-operation-types.ts by removing the trailing empty line from the source emitter. No generated file was edited.

Verification

• pnpm --filter @cloudflare/forge-sdk-ts test — 21 passing tests.
• pnpm --filter @cloudflare/forge-sdk-ts typecheck:scripts and changed-file format checks — pass.
• The transformer package was rebuilt from this exact source. Full Docker-backed generation from a two-GET OpenAPI projection then completed twice using Fern TypeScript 3.80.1. Both runs produced 62 files and two mapped GET operations. Complete relative path/size/SHA-256 manifests matched byte for byte (manifest SHA-256 1c1c2dcf2c21cdcb840de0ad727a03c159995b22318568f3245781081caa6faf); full output directory diff was empty.
• git diff --check on the source branch — pass. The generated operation-types file now ends with a single newline.

These changes do not alter generated method selection, response typing, or runtime request behavior.