Track dropped extension telemetry reports

## Summary
Add observability for extension telemetry reports that are dropped or rejected by the host.

## Problem
Extensions can lose telemetry because values exceed limits, keys are invalid, the install source is ineligible, the event budget is exhausted, or another validation rule rejects the report. Today there is no aggregate signal showing that an extension is failing to report expected data.

## Requirements
- Define a low-cardinality metric or equivalent signal for dropped/rejected reports.
- Capture a bounded reason/category rather than caller-controlled content.
- Avoid creating a telemetry loop or high-cardinality dimensions.
- Document how extension owners can use the signal to diagnose reporting issues.
- Add tests for the relevant drop paths.

## Reference
Follow-up from [PR #9174 review comment](https://github.com/Azure/azure-dev/pull/9174#discussion_r3754696838).