docs(sdk): clarify ACP context-fill metrics

[jiuyueQ]

• 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

Documentation claim	Source evidence
The server's context window comes from UsageUpdate.size.	ACP bridge
Context fill uses UsageUpdate.used, including updates without response token counts; input/output is the fallback without an update.	ACP usage recording and default metrics calculation

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