fix: prevent self-amplifying compaction failure loop (#900)

[kartikmengane09-oss]

Summary

Fixes the self-amplifying compaction failure loop described in #900.

Problem

Long-running sessions can exceed the model context window. The initial provider call fails, recovery compaction is then attempted with an oversized summarization request, and the compaction failure is persisted and replayed into subsequent model context. This causes repeated failures and progressively larger context until the session becomes unresponsive.

Changes

• Prevent internal failed/recovery state from contaminating model-facing context while preserving the durable JSONL audit trail.
• Bound compaction/summarization requests against the available model context budget.
• Process oversized history through bounded message-aligned summarization rather than a single oversized request.
• Ensure necessary pre-compaction checks are performed before provider calls, including direct agent-message paths.
• Handle stale provider usage with a current-state token estimate where necessary.
• Prevent uncontrolled repeated compaction failures and preserve valid state when compaction fails.

Validation

Relevant compaction and session-context regression tests are included/run as part of the change.

Closes #900

Note

Fix self-amplifying compaction failure loop by adding context budget enforcement

• compaction.ts: generateSummary now splits oversized conversations into budget-sized chunks using a chars/4 token estimate, summarizes each chunk independently, then aggregates into a final summary with a second-pass if needed.
• compact validates the produced summary against the model's context window after generation and throws if the result would exceed the safe limit, preventing an oversized compacted state from being committed.
• session-manager.ts: buildSessionContext now filters out assistant messages with stopReason: "error" and compaction_outcome custom messages from the model context to avoid replaying failed attempts.
• Behavioral Change: compact now throws instead of silently succeeding when the compacted result is too large for the context window.

^{Macroscope summarized 6f1ef3e.}

[sethkarten]

Thank you for the report and proposed work. This root cause is now covered by maintainer-owned stacked PR #1165, authored independently from upstream/main.

We did not inspect or reuse this PR's diff, branch, commits, implementation code, or tests; its public description/comments were used only as a bug report. To keep one review surface, this PR is superseded by #1165 and is being closed.

The complete review stack is #1158–#1165. It is being left unmerged for human review after CI and review-bot findings are cleared.