fix(opencode): bound compaction request payload

[timrichardson]

Issue for this PR

Closes #29857

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

This branch changes /compact so very large sessions are more likely to compact successfully instead of overflowing while trying to summarize.

The compaction path now builds a bounded model payload. It computes a compaction budget as floor(usable(...) * 0.85), where usable(...) is opencode's safe history budget for the selected model:

• If the model has limit.input, usable(...) is limit.input - reserved.
• Otherwise it is limit.context - maxOutputTokens.
• reserved comes from config.compaction.reserved, or defaults to min(20_000, maxOutputTokens).
• Models with limit.context === 0 keep the existing overflow-disabled behavior.

For extra safety, compaction first performs a local preflight using the legacy payload shape: media stripped, reasoning preserved, and tool output capped at 2_000 chars. If that estimate fits the compaction budget, /compact sends the legacy payload unchanged.

If the legacy payload would exceed the budget, /compact falls back to a reduced payload. In that fallback it strips reasoning parts, including large provider metadata like encrypted reasoning blobs, and adaptively truncates tool outputs by binary-searching from 0 to 2_000 chars and keeping the largest preview that still fits the budget.

This also fixes toolOutputMaxChars: 0: on dev, 0 was treated as "no limit"; on this branch, only undefined means "no limit", so 0 can fully remove tool output content while keeping the truncation marker.

Net effect: before, /compact could fail because the summarization request itself was too large. Now it preserves the old payload when safe, and only sends a reduced, budget-aware history when needed, so sessions with huge tool outputs or reasoning metadata can be summarized rather than immediately overflowing.

How did you verify your code works?

• bun test test/session/compaction.test.ts (55 pass, 1 skip)
• bun test test/session/message-v2.test.ts test/session/revert-compact.test.ts (45 pass)

The compaction tests cover:

• overflow threshold behavior, including limit.input, output headroom, cache reads, disabled auto-compaction, and unknown context limits
• compaction marker creation and summary process behavior
• tool-output pruning, including protected skill tool output
• auto-continue, overflow replay, repeated compactions, summary anchoring, abort handling, and no tool calls during summary generation
• the new bounded payload path preserving the legacy payload when it fits
• the new reduced payload path stripping reasoning metadata, truncating tool output, and staying within the calculated compaction budget
• irreducibly oversized payloads failing cleanly by marking the summary message as errored
• plugin-injected compaction context being included in budget sizing
• message conversion behavior for compacted and truncated tool output
• revert plus compact workflows

I also verified the anonymized fixture from #29857 locally:

• legacy payload shape: 788327 estimated tokens, overflows
• fixed payload shape: 308291 estimated tokens, compacts successfully

Note: local bun typecheck / pre-push typecheck is currently blocked by existing event-stream typing errors outside this compaction change (stream.transport.ts, TUI event/sync, desktop server-sdk.tsx). The test commit was pushed with hook bypass after explicit maintainer authorization.

Screenshots / recordings

N/A

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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