Replace oxfmt with prettier for code formatting

[pyramation]

Summary

Replaces the experimental oxfmt formatter with prettier in the @constructive-io/graphql-codegen package. This change bundles prettier with sensible default formatting options that are applied to generated SDK code.

Key changes:

• Swapped oxfmt dependency for prettier (already available in workspace)
• Added bundled config file with defaults: singleQuote: true, trailingComma: "all", tabWidth: 2, semi: true
• Updated formatOutput function to use prettier with the bundled config
• Added minimal test that verifies prettier runs and applies formatting
• Updated Jest snapshot headers to new jestjs.io URL format

Note: Generated code will now use single quotes instead of double quotes (oxfmt's default was singleQuote: false). This is intentional per requirements.

Review & Testing Checklist for Human

• Verify config file is included in dist: The codegen-prettier.json file is in src/cli/commands/ and resolved via __dirname. Confirm the build process copies this to the dist folder correctly.
• Test end-to-end codegen: Run graphql-codegen generate on a real schema and verify the output files are formatted with single quotes and semicolons.
• Verify prettier binary path: The code uses prettier/bin/prettier.cjs - confirm this works when the package is installed as a dependency.

Recommended test plan:

1. Build the package: pnpm build in graphql/codegen
2. Run codegen against a test schema and inspect the generated output
3. Verify formatting matches expected config (single quotes, trailing commas, semicolons)

Notes

• Link to Devin run: https://app.devin.ai/sessions/19149b813cda4e97ba9e8ec1bfd00676
• Requested by: @pyramation

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