fix(error_reporting): respect quota_project configuration

This PR fixes an issue where the Google::Cloud::ErrorReporting client was completely ignoring the quota_project configuration parameter, causing API calls to default billing to the project associated with the credentials. This resulted in PermissionDeniedError (Error Reporting API not enabled) for users attempting to report errors to a target project that was different from their credentials project (e.g., issue #25862).

closes: #25862

Changes

• Respect Configuration: Updated Google::Cloud::ErrorReporting.new to correctly resolve quota_project from the library configuration and pipe it down to the internal Service layer.
• Service Wrapper: Updated Google::Cloud::ErrorReporting::Service to accept quota_project (falling back to credentials) and apply it to the underlying gRPC client configuration.
• Documentation: Added multi-project configuration examples and descriptions to README.md and AUTHENTICATION.md.
• Testing: Added unit test coverage to confirm configuration propagation and refined YARD doctest mock expectations.
• Modernization: Modernized Google::Cloud::ErrorReporting::Project#report to use Ruby 3.2+ anonymous block forwarding (*args, &), resolving pre-existing RuboCop style offenses.

Expectation Setting: Target Project vs. Quota Project

In Google Cloud APIs, the Target Project (where resources are stored/written) and the Quota Project (which project is billed for the API call) are intentionally separate concepts:

• By default, the library bills the project associated with your credentials.
• If you need to report errors to a project that differs from your credentials project, you must explicitly configure quota_project via library configuration or the GOOGLE_CLOUD_QUOTA_PROJECT environment variable.

Configuration Example:

Google::Cloud::ErrorReporting.configure do |config|
  config.project_id    = "target-project-id"   # Where error events are ingested
  config.quota_project = "billing-project-id"  # Project billed for the quota
  config.credentials   = "path/to/keyfile.json"
end

# Client will now respect the configured quota project for billing
client = Google::Cloud::ErrorReporting.new