fix(s08): prevent active request loss after snip_compact

[ship-onward]

Summary

Fixes #577

snip_compact() may archive the current user request during a long tool-use loop,
causing the agent to fall back to an older request.

This PR ensures that the active request remains visible after compaction.

Changes

• Preserve the current active_request during snip_compact
• Add a regression test for long tool-use conversations
• Keep the existing context compaction flow unchanged

Testing

• Added/updated tests/test_s08_context_compact.py
• Reproduced the original three-prompt scenario
• Confirmed Prompt 3 no longer falls back to Prompt 1
• Existing behavior remains unchanged

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