feat(client): support opaque payload schemas

[opencode-agent]

Summary

• keep struct payloads flattened while projecting every non-struct payload as a required payload input field
• send opaque payload values directly in Promise and Effect request bodies, with a targeted collision error
• cover union, array, primitive, and index-signature record classification across generated surfaces and the strict consumer fixture

Design

• opaque field name: payload
• opaque classification: any decoded payload that is not an object with zero index signatures
• mcp.add can now be unwrapped in a follow-up; this PR intentionally leaves its protocol unchanged

Validation

• bun test --timeout 5000 --only-failures (packages/httpapi-codegen)
• bun typecheck (packages/httpapi-codegen)
• bun run generate (packages/client), with no diff under packages/client
• bun test --timeout 5000 (packages/client)
• bun typecheck (packages/client, packages/sdk-next, packages/cli, packages/plugin, packages/tui)

Requested by: @rekram1-node (Aiden Cline via Slack)