docs(sdk): clarify ACP context-fill metrics - #922

• I have read and reviewed the documentation changes to the best of my ability.
• If the change is significant, I have run the documentation site locally and confirmed it renders as expected.

Summary of changes

Document how ACP context-fill metrics use UsageUpdate.used and size, including the no-response-usage case and the input-plus-output fallback. Clarify why whole-turn token totals remain separate accounting values.

Companion to software-agent-sdk #5692, required by the SDK documentation workflow. Please coordinate merging with the SDK fix; these paragraphs describe that PR's behavior.

Validation

• Compiled the full changed guide with @mdx-js/mdx 3.1.1; passed.
• Confirmed the added /sdk/guides/metrics link resolves to an existing guide and git diff --check passes.
• Verified behavioral claims against SDK commit 2d9dd1cb45103d6eb1dfdb65a35aebc18b50c582, its 534 passing Linux tests, and a real offline ACP subprocess through Conversation.run().
• A Mintlify browser preview and full-site broken-link scan were not run locally. Existing examples were not changed.

Source Verification

Prepared and validated with AI assistance (Codex). The template's human review checkboxes are left for the contributor.