Fix OpenCode raw text delta assembly

[Kiri110K]

What changed

• Treat OpenCode message.part.delta text chunks as raw incremental deltas and append them exactly as received.
• Keep message.part.updated snapshot handling as the reconciliation path.
• Add regression coverage for coincidental suffix/prefix overlap, where the old dedupe logic could strip valid leading characters from a delta.

Why this should exist

Fixes #2380.
Likely related to #2379.

OpenCode text deltas are already valid incremental chunks. The previous overlap heuristic tried to dedupe suffix/prefix overlap between the already assembled assistant text and the next delta, but normal text can overlap coincidentally.

For example, if the current text ends in B and the next raw delta is Bonus, stripping the overlap turns Bonus into onus. A later message.part.updated snapshot then reconciles the missing text by emitting a larger corrective suffix, which can make the assistant response appear duplicated or corrupted near completion.

This change makes raw deltas authoritative and leaves full part snapshots to reconcile state without mutating normal streamed chunks.

Scope

• OpenCode provider assistant text assembly only.
• No UI change.
• No protocol or persistence change.
• No behavior change for other providers.

Validation

• bun fmt
• bun lint - 0 errors, existing warnings only
• bun typecheck - passed
• bun run --cwd apps/server test src/provider/Layers/OpenCodeAdapter.test.ts - 14 passed

UI Changes

Not applicable. This is a server-side provider streaming fix.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes (N/A)
• I included a video for animation/interaction changes (N/A)

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Note

Medium Risk
Changes OpenCode assistant text streaming assembly; incorrect handling could impact user-visible output ordering/duplication, but scope is limited to a single provider and covered by new regression tests.

Overview
Fixes OpenCode assistant text assembly by treating message.part.delta text as raw incremental chunks and appending them verbatim, removing the prior suffix/prefix overlap de-duplication heuristic.

Keeps message.part.updated handling as the reconciliation mechanism, and updates/adds tests to cover overlapping-update scenarios and the regression where coincidental prefix overlap (e.g. B + Bonus) previously caused valid characters to be stripped.

^{Reviewed by Cursor Bugbot for commit 584ab1c. Bugbot is set up for automated code reviews on this repo. Configure here.}

Note

Fix appendOpenCodeAssistantTextDelta to append raw deltas without stripping prefix overlaps

• Removes the suffixPrefixOverlap helper and the trimming logic from appendOpenCodeAssistantTextDelta in OpenCodeAdapter.ts; deltas are now appended as-is.
• Behavioral Change: nextText is now a direct concatenation of previousText and the full delta, and callers receive the original delta string rather than an overlap-trimmed version.

^{Macroscope summarized 584ab1c.}

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Approvability

Verdict: Approved

Straightforward bug fix that removes incorrect text deduplication logic which was stripping valid content. The change is small, self-contained, and includes comprehensive test updates verifying the correct behavior.

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