fix: reuse parent system prompt for background review to preserve cache bytes

[NeroNarada]

Summary

Background self-improvement review currently builds a new AIAgent with a freshly
generated system prompt, which can differ from the parent agent and break prefix
cache / preprocessing tree-dedup behavior. This change reuses the parent’s
already-built system prompt when spawning the review agent, while keeping the
review toolset restricted to memory/skills.

Changes

• Add optional cached_system_prompt: Optional[str] = None to AIAgent.__init__.
• Seed self._cached_system_prompt from that value during initialization.
• In _spawn_background_review, pass:
  • session_id=self.session_id
  • pass_session_id=self.pass_session_id
  • cached_system_prompt (from parent cache, or built lazily if absent)
• Keep enabled_toolsets=["memory", "skills"] unchanged.
• Add regression coverage in background review tests:
  • review fork receives inherited system prompt and session settings,
  • fallback to a parent-built prompt when cache is absent.

Motivation

This preserves byte-identical prompt bytes across the parent/review boundary
without relaxing the tool-restriction safety model from #15204.

Issue

• Fixes [Bug]: Background self-improvement review's fresh AIAgent generates a system prompt that bytes-differs from the parent, busting prompt cache + preprocessing tree-dedup #25322

[alt-glitch]

This PR appears to be superseded by #25332, which contains all changes from this PR plus additional gateway session cleanup and tool-defs cache cap fixes. Additionally, #17276 is an existing open PR addressing the same background review prompt cache issue (#25322). Consider closing in favor of #25332.

[NeroNarada]

Superseded by #25332, which carries the same intent plus gateway session cleanup/tool-defs cache cap fixes. Keeping only the broader follow-up.