feat: add decompress option support with Fastly decompressGzip mapping

[trieloff]

Summary

Implements cross-platform decompression control by adding support for the decompress option in the fetch polyfill, which automatically maps to Fastly's decompressGzip option when running on Fastly Compute. This resolves issue #81.

Changes

• Modified src/template/polyfills/fetch.js:

  • Created fetch wrapper that detects runtime environment (Fastly vs Cloudflare/Node.js)
  • Maps decompress: true|false option to platform-specific behavior:
    • Fastly: Automatically sets fastly.decompressGzip based on decompress value
    • Cloudflare: Pass-through (auto-decompresses by default)
    • Node.js: Pass-through to @adobe/fetch (handles decompress natively)
  • Explicit fastly options take precedence over automatic mapping
  • Converted module to ES modules for consistency with other template files

• Added test/fetch-polyfill.test.js:

  • 10 comprehensive test cases covering all scenarios
  • Tests for Fastly environment: default behavior, explicit options, overrides, merging
  • Tests for non-Fastly environments: pass-through behavior
  • 97% code coverage

Usage Example

import { fetch } from '@adobe/fetch';

// Default behavior (decompress: true)
const response = await fetch(url);

// Explicit decompression control
const response = await fetch(url, { decompress: false });

// Explicit fastly options (override)
const response = await fetch(url, {
  decompress: true,
  fastly: { decompressGzip: false, backend: 'custom' }
});

Test plan

• Unit tests added and passing (10 tests, 97% coverage)
• Fastly environment behavior verified
• Non-Fastly environment behavior verified
• ES module exports compatible with bundler
• Conventional commit message used

Related Issues

Closes #81

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 96.15385% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/template/polyfills/fetch.js	96.15%	2 Missing ⚠️

📢 Thoughts on this report? Let us know!

[github-actions]

This PR will trigger a minor release when merged.

[trieloff]

what year is it?

[trieloff]

hey!

[trieloff]

depends on #88 for integration tests to run on cloudflare