fix(openai-history): keepTail:0 in mixed Responses collapse protected every message

[dex0shubham]

Problem

In planResponsesMixedCollapse (src/core/openai-history.ts), the protected trailing messages are computed as:

messageIndices.slice(-Math.max(0, Math.floor(o.keepTail)))

When keepTail is 0, that expression is slice(-0). Array.prototype.slice(-0) treats -0 as 0 and returns the entire array — so "protect zero trailing messages" instead protects every message. Every old turn then becomes a hard barrier, no history collapses, and the planner returns reason: 'no_closed_prefix' with nothing imaged (the exact inverse of the intent).

keepTail: 0 is a legal, reachable config: it flows through nonNegativeInt(historyIn?.keepTail, …) (which accepts 0) from a host's gptHistory.keepTail override. The shipped Sol profile uses keepTail: 1, so this is a latent correctness bug on an override path, not a default-path failure.

Fix

Guard keepTail === 0 explicitly so it protects nothing; the latest user turn is still protected separately via latestUserIndex.

const tail = Math.max(0, Math.floor(o.keepTail));
const protectedMessages = new Set(tail > 0 ? messageIndices.slice(-tail) : []);

Added a regression test (verified it fails on the old code, passes on the fix). Full suite green except 3 pre-existing Windows-only file-permission failures in node-security.test.ts (unrelated; fail on a clean tree here too). typecheck clean.

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