[observability] Add estimated usage fallback

[mfittko]

Summary

Add estimated usage fallback when upstream responses omit explicit usage data.

Changes

• add fallback token counting for streamed chat-completions responses without usage chunks
• add fallback request token counting for responses-style error payloads without usage
• add SSE response-content reconstruction needed for that estimation path
• add the proxy and usage-event-builder coverage for the fallback behavior

Scope

PR #1 stays focused on usage reported directly by the upstream API.

This PR adds the separate heuristic path for cases where upstream usage is missing.

Testing

• bundle exec rspec spec/usage_event_builder_spec.rb spec/proxy_spec.rb
• make coverage
• make lint