feat(mcp): forward plugin request metadata

[dialupdisaster]

Issue for this PR

Closes#17084

Updates the implementation proposed in #21539 and #21624 for the current MCP tool paths.

Type of change

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

What does this PR do?

Plugins can now set optional _meta fields in tool.execute.before. OpenCode forwards them to downstream MCP tools/call requests without exposing them as tool arguments.

The forwarding covers both normal MCP tools and code-mode child calls. A shared call path preserves MCP SDK 1.29's own metadata, including progress tokens, and supports SEP-414 trace context without adding metadata by default.

How did you verify your code works?

• bun typecheck in packages/opencode and packages/plugin
• root bun typecheck (30 packages)
• env -u OPENCODE_DISABLE_CLAUDE_CODE bun test in packages/opencode (3,199 pass, 0 fail)
• bun run test:httpapi in packages/opencode (208/208 in coverage, auth, and effect modes)
• focused normal, code-mode, and in-memory MCP transport tests

bun lint still reports the existing octal-escape error in packages/session-ui/src/v2/components/prompt-input/index.tsx; changed files have no lint errors.

Screenshots / recordings

Not applicable; no UI changes.

Checklist

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

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on my search, I found related PRs that should be reviewed for context:

Potentially Related PRs:

1. feat(mcp): forward session ID and plugin _meta to MCP tool calls #21624 - feat(mcp): forward session ID and plugin _meta to MCP tool calls

   • This appears to be a previous implementation proposal that the current PR (feat(mcp): forward plugin request metadata #38579) updates, as mentioned in the PR description ("Updates the implementation proposed in feat(mcp): forward plugin-set _meta to downstream MCP tool calls #21539 and feat(mcp): forward session ID and plugin _meta to MCP tool calls #21624")

2. feat(mcp): add mcp.call.before plugin hook for per-call MCP request headers #28319 - feat(mcp): add mcp.call.before plugin hook for per-call MCP request headers

   • Related feature addressing similar MCP request customization through plugins

These are not duplicates but rather related/predecessor work. The current PR (#38579) is an updated implementation of the approach outlined in #21624 adapted for current MCP tool paths.