[P2b] feat: messages.transform E2E tests

[four-bytes-robby]

Purpose

Validate that experimental.chat.messages.transform correctly parses compaction signals from assistant messages, strips them from visible output, and compacts message history.

Technical Details

• Mock message history with assistant messages containing compaction_signal blocks
• Assert: signal is parsed and stored in session state
• Assert: compaction_signal is stripped from visible output
• Assert: empty assistant messages get placeholder (…) injected
• Assert: message history is compacted (old tool outputs removed/truncated)
• Assert: providerID/modelID are extracted from user message info
• Assert: compactMessageHistory is called with correct sessionID

Files to Create

• test/e2e/messages-transform.test.ts
• test/fixtures/sample-messages.ts

Acceptance Criteria

• Tests pass with bun test test/e2e/messages-transform.test.ts
• Signal parsing, stripping, and history compaction tested
• Edge cases: no signal, multiple signals, empty messages