feat(acp): session rotation on context window exhaustion

[wesbillman]

Summary

When an ACP agent hits MaxTokens or MaxTurnRequests (context window exhaustion), the session was previously returned to the pool unchanged — subsequent prompts would re-enter the bloated session and fail or degrade. This PR adds automatic session rotation to handle context window exhaustion gracefully.

Changes

Reactive Session Rotation (pool.rs)

• When StopReason::MaxTokens or StopReason::MaxTurnRequests is returned, the channel session is invalidated so the next prompt creates a fresh one
• Heartbeat sessions are also rotated on exhaustion

Proactive Turn-Based Rotation (config.rs, pool.rs, main.rs)

• New config: SPROUT_ACP_MAX_TURNS_PER_SESSION (CLI: --max-turns-per-session, default: 0 = disabled)
• When set, sessions are rotated after N successful turns, preventing context buildup before hitting the wall
• Set to 1 for "fresh session per task" mode — agents use channel history (context_message_limit) as context instead of accumulating in-session state

Refactored Session Invalidation (pool.rs)

• Extracted OwnedAgent::invalidate_session() and OwnedAgent::invalidate_all_sessions() helper methods
• Replaced all scattered sessions.remove() / sessions.clear() / heartbeat_session = None patterns throughout run_prompt_task() with the new helpers
• Turn counters are now consistently cleaned up across all invalidation paths (error, timeout, exit, rotation)

Logging

• MaxTokens and MaxTurnRequests log messages now note upcoming session rotation
• Session rotation events logged at info level with source and stop reason

Files Changed

• crates/sprout-acp/src/pool.rs — rotation logic, turn counters, invalidation helpers, PromptContext field
• crates/sprout-acp/src/config.rs — new CLI arg, Config field, summary string
• crates/sprout-acp/src/main.rs — wired new fields into all 3 OwnedAgent construction sites and PromptContext

Testing

• All 172 existing unit tests pass
• Clean clippy, clean fmt
• Pre-push CI (rust-fmt, rust-clippy, rust-tests, desktop-build, desktop-check, desktop-tauri-check) all pass
• Backward compatible: default max_turns_per_session=0 preserves existing behavior exactly

[tlongwell-block]

Pushed a fix commit on top: 0e55e3d — consistent turn counter cleanup + SessionState extraction.

What changed:

The PR correctly migrated the pre-prompt error paths in run_prompt_task() to use invalidate_session() / invalidate_all_sessions(), but four post-prompt error paths still used raw sessions.clear() / sessions.remove() without clearing turn counters. After a post-prompt error, the replacement session would inherit a stale turn count and could rotate prematurely when max_turns_per_session is enabled.

The fix:

• Extracted SessionState from OwnedAgent — holds sessions, turn_counts, heartbeat_session, heartbeat_turn_count and the invalidation methods. This separates connection state from session state and makes the state machine unit-testable without spawning a real agent subprocess.
• Three methods on SessionState: invalidate(source), invalidate_channel(id) -> bool, invalidate_all(). All session mutation in pool.rs and main.rs now goes through these — zero raw field access outside the impl.
• Fixed handle_prompt_result in main.rs: the removed_channels retain path was also missing turn counter cleanup. Now uses invalidate_channel() in a loop.
• 8 unit tests covering all SessionState methods and edge cases.
• 180 tests pass, clean clippy, clean fmt, all pre-push hooks green.

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This comment was written by AI (goose).