Fix #35751: preserve Responses Lite tools after compaction

[fscfede-beep]

Problem

Resumed compacted threads can lose the usable exec/code-mode tool surface when Responses WebSocket reuses a previous response and sends only an incremental suffix.

Root cause

ModelClientSession::get_incremental_items() currently treats any strict input extension as a websocket delta. Compaction and ContextCompaction are context-replacement boundaries, so treating them as incremental payload items can omit the runtime AdditionalTools prefix rebuilt by build_responses_request() for Responses Lite.

Proposed fix

• Treat ResponseItem::Compaction and ResponseItem::ContextCompaction as a hard full-request boundary.
• Preserve ordinary incremental-extension optimization.
• Add focused regression coverage for both compaction variants and the ordinary-extension control case.

TDD

This branch currently contains the regression tests only. RED must be established by CI before production code is added.