[wrangler] fix: improve R2 CORS validation and error messaging

[venkatnikhilm]

This PR addresses #8486 by adding local validation to the r2 bucket cors set command.

Previously, users providing AWS S3-formatted CORS files (using CORSRules or PascalCase keys like AllowedOrigins) would receive a generic 10040 API error from the server. This PR intercepts these common formatting mistakes locally, providing a descriptive error message and a link to the correct Cloudflare R2 documentation.

Validation conducted:

• Manual Test (AWS Format): Verified that a file containing CORSRules or AllowedOrigins triggers the new custom error message.
• Manual Test (R2 Format): Verified that a correctly formatted R2 CORS file passes validation and proceeds to the API call.
• Linting/Types: Successfully ran pnpm run check --filter wrangler in the project root with 0 errors.

Changes:

• Modified packages/wrangler/src/r2/cors.ts to include a validation layer using parseJSON.
• Added detection for AWS-specific top-level keys and PascalCase property keys.
• Included an official documentation link in the error output.

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[devin-ai-integration]

Devin Review found 1 potential issue.

View 3 additional findings in Devin Review.