Implement mappings.json parsing and validation #1077

Context

The CLI needs to parse and validate the user-provided mappings JSON file before sending it to the API.

What

• Parse JSON array of {stackRoot, sourceRoot} objects from the file path argument
• Validate structure: must be an array, each element must have stackRoot and sourceRoot strings
• Provide helpful error messages for malformed files (invalid JSON, missing fields, wrong types)
• Validate max 200 mappings (matching backend limit)

Expected file format:

[
  {
    "stackRoot": "com/example/android/maps",
    "sourceRoot": "modules/maps/src/main/java/com/example/android/maps"
  }
]

Repo

This is in the sentry-cli repository.