fix(opencode): drop empty text separators between signed reasoning blocks

[vladislav-miroshnikov]

Issue for this PR

Closes #41738

Type of change

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

What does this PR do?

Anthropic signs thinking/redacted_thinking reasoning blocks and rejects any modification to the assistant message that contains them. Empty text parts ("") can appear as structural separators between reasoning groups (e.g. step-start, reasoning(sig), text(""), step-start, reasoning(sig)). Previously these were rewritten to a single space (" ") to survive AI SDK filtering, but that mutation itself causes Anthropic to reject the signed reasoning blocks as "modified" — breaking compaction for any session using extended thinking.

This drops empty text parts entirely when signed reasoning is present. Step-start parts already delimit reasoning groups, so the separator is not needed. The signed blocks remain byte-for-byte identical to the original response.

How did you verify your code works?

• cd packages/opencode && bun test test/session/message-v2.test.ts — 37 pass, 0 fail
• cd packages/opencode && bun test test/session/compaction.test.ts — 52 pass, 1 skip, 0 fail
• cd packages/opencode && bun typecheck — clean
• Changed-file Prettier and git diff --check

Two new tests verify that signed reasoning and redacted thinking blocks survive toModelMessages with their signatures intact and no mutated text separator.

Checklist

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

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