fix(cli): preserve ACP reasoning boundaries and effort selections

[nexxeln]

Summary

• Give each reasoning part a consistent ACP message ID during streaming and transcript replay.
• Preserve effort when synchronizing the same model, while honoring explicit variant changes.
• Display unset or explicit default effort as Default and allow selecting it without a named provider variant.

V2 follow-up to #48225. Related to #31961.

Testing

• Extend existing streaming/replay and real ACP subprocess tests; no additional test cases.
• bun test test/acp --timeout 30000 --only-failures in packages/cli: 96 passed.
• Pre-push workspace typecheck: all 35 tasks passed.
• Prettier and git diff --check passed.