feat(acp): plumb structured output format through _meta

[AbhiShake1]

What does this PR do?

Forwards the structured-output format option through the ACP prompt() handler so ACP clients (IDEs, taskflow harnesses, other ACP-speaking tools) can request structured JSON output — the same capability the SDK gained in #8161.

Before this change, the SDK honored session.prompt({ format }) but the ACP handler silently dropped it, leaving ACP clients unable to access structured output over the protocol. Motivated by the same use cases as #9320, #10456, and #5639.

Changes

• Read params._meta?.opencode?.format in Agent.prompt(), validate it against the existing MessageV2.Format zod schema (the same one feat: support claude agent SDK-style structured outputs in the OpenCode SDK  #8161 added), and forward it to this.sdk.session.prompt({ ..., format }).
• Echo the resulting msg.structured value back on the PromptResponse._meta as { opencode: { structured } } when the model returned structured output.
• Uses ACP's standard _meta extension point with opencode's existing _meta.opencode.* namespacing convention (see buildVariantMeta at the same file).

How did you verify your code works?

• bun run typecheck (tsgo --noEmit) — clean.
• bun test test/acp/agent-interface.test.ts — 1 pass, confirming Agent still implements the ACPAgent interface.
• Manual verification path: send an ACP prompt request with _meta: { opencode: { format: { type: "json_schema", schema: { ... } } } } and observe response._meta.opencode.structured containing the validated JSON.

Notes

• Invalid format payloads silently fall through to plain-text mode (safeParse + optional chaining) — matches the SDK's format.optional() contract.
• Only the non-command branch is wired; slash commands (/foo) don't consume structured output.

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