## Stack

Part 2 of the split. #64047 is merged, and this PR now targets `main`.
Current main was merged into this branch without rewriting history. The
diff contains only counting/consent work and the required README review
acknowledgment (8 files, +837/-80). The Copilot metadata sibling #64195
does not depend on this PR.

## Summary

Add optional provider-backed input counting for direct Anthropic, direct
OpenAI Responses, and hosted Anthropic/OpenAI models. Preserve
structured input and reuse provider conversion, authentication refresh,
and client request limiting. Unsupported providers return `None`;
provider and malformed-response errors remain visible.

Keep the existing hosted data-retention consent rule common to counting,
native compaction, and generation. Counting never sends input before
consent. The matched Cloud endpoint support is merged; its deployment
remains a prerequisite for downstream callers.

This PR does not change Copilot prompt/context metadata, compaction
thresholds, or tooltips. No chunking or paid tests are introduced.

## Validation

- After resolving the main merge: 226 tests passed across anthropic,
open_ai, language_models, and language_models_cloud. Main’s consolidated
output-limit tests and renamed serialization test are preserved.
- Scoped script/clippy passed for anthropic, open_ai, language_models,
and language_models_cloud.
- Formatting and whitespace checks passed.
- Coverage includes images, structured request fields, authenticated
hosted requests, unsupported providers, error propagation, and consent
rejection before HTTP.
- No live provider calls were made.

Release Notes:

- Fixed data-retention consent checks for hosted counting and compaction
requests.