port(upstream): #3506 - harden Claude trailing prefill and tool types

[bloodf]

Summary

• What changed: Ports upstream #3506 and covers stacked #3484. Adds shared Claude assistant-prefill policy across translated and native Claude paths, passes native request headers into Claude passthrough normalization, and preserves declared Claude/server tool types instead of overwriting them with the fallback. Includes compatibility regressions.
• Why it changed: Claude requests ending in assistant prefill need consistent continuation handling, while callers that explicitly request preservation must retain the terminal assistant turn. Typed/server tools must remain typed rather than falling back to custom.

Type

Select one (conventional commit prefix):

• feat: — new feature
• fix: — bug fix
• refactor: — code change that neither fixes a bug nor adds a feature
• docs: — documentation only
• ci: — continuous integration / build / deployment
• chore: — maintenance, tooling, or other non-code change

This upstream port uses the repository's port(upstream): commit convention.

Breaking changes

• No
• Yes — describe below:

Testing

• What was tested: Focused translator compatibility coverage for translated and native Claude assistant-prefill policy; focused native Claude tool-type fallback coverage; existing cache-control and reasoning-history compatibility cases.
• Commands run: Canonical gate and focused coverage at exact head 05bb4417cbd68c91413bf666e9609fc9209b14f9; commitlint, docs, registry, and build checks all passed.
• Evidence (logs, screenshots, output links): Exact branch head 05bb4417cbd68c91413bf666e9609fc9209b14f9. Canonical gate: PASS under Node v20.20.2, ABI 115; focused coverage: 5 files / 68 tests passed; full npm run test:ci: RC 0, raw / known / stale failures 0, baseline additions 0; lint / docs / registry / build: PASS. Pinned judge judge-580-current: APPROVED, no blockers.

Files, docs, and changelog

• Runtime: open-sse/handlers/chatCore.js, open-sse/translator/concerns/assistantPrefillPolicy.js, open-sse/translator/formats/claude.js.
• Tests: tests/translator/assistant-prefill-policy.test.js, tests/translator/port-2401-reasoning-history-bridge.test.js, tests/unit/claude-cache-control.test.js, tests/unit/force-stream-config.test.js.
• Docs: docs/reference/api.md.
• Changelog: CHANGELOG.md.

Existing PRs

Searched current repository open and closed PRs for #3506, Claude trailing prefill, tool types, and campaign/issue-580. No duplicate implementation PR found. Search returned only unrelated #291 (merged Claude Copilot native routing) and #95 (closed server-tool literal names).

Authoring environment

• Model: durindoor/cx/gpt-5.6-terra.
• Harness: Oh My Pi. Harness version: not exposed by this session.
• Installed plugins: no plugin inventory exposed to this session.
• Human involvement: branch contents and requested PR metadata were supplied by the human partner; this draft awaits human review of the complete diff before submission.

Checklist

• Tests added or updated for behavioral changes
• No new dependencies without explicit justification
• Conventional commit used (port(upstream):; validated by commitlint)
• CHANGELOG updated if this change is user-facing
• No secrets, keys, credentials, or tokens committed

Closes #580