Add body_after_prefix auto-compact token limit scope

[jif-oai]

Why

model_auto_compact_token_limit has only been able to budget the full active context. That makes it hard to set a small "growth since compaction" budget for sessions that preserve a large carried window prefix: the preserved prefix can consume the whole budget and force immediate repeated compaction.

This PR adds an opt-in body_after_prefix scope so callers can apply model_auto_compact_token_limit to sampled output and later growth after the current carried prefix, while still forcing compaction before the full model context window is exhausted.

What changed

• Adds AutoCompactTokenLimitScope with the existing total behavior as the default and a new body_after_prefix mode: config_types.rs.
• Threads model_auto_compact_token_limit_scope through config loading, Config, core-api, and app-server v2 schema/TypeScript generation.
• Records the first observed input-token count for a body_after_prefix compaction window and uses it as the baseline when deciding whether the scoped auto-compaction budget is exhausted: turn.rs.
• Keeps a hard context-window cap in body_after_prefix, so scoped budgeting cannot let the active context overrun the usable window.

Verification

Added compact-suite coverage for the two key behaviors: body_after_prefix does not re-compact just because the carried prefix is larger than the scoped budget, and it still compacts when the total active context reaches the configured context window: compact.rs.

[jif-oai]

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