fix(opencode): pass mcp_timeout as AI SDK streamText stepMs to prevent 120s timeout

[claude-code-best]

The AI SDK streamText() defaults stepMs to 120s, which covers the entire step including tool execution. This caused MCP tool calls to timeout at 120s even when experimental.mcp_timeout was configured to a much larger value, because the AI SDK step timeout fired before the MCP-level timeout.

Type of change

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

What does this PR do?

MCP tool calls timeout at 120s regardless of experimental.mcp_timeout config.

The AI SDK streamText() has a stepMs param (default 120s) that covers the entire step — LLM call + tool execution. OpenCode correctly passes mcp_timeout to MCP SDK's callTool(), but never sets stepMs on streamText(). So the AI SDK's 120s default fires before the MCP-level timeout is ever reached.

Fix: pass experimental.mcp_timeout as stepMs to streamText(), so the AI SDK step timeout aligns with the MCP timeout. When mcp_timeout is not configured, behavior is unchanged (AI SDK default applies).

How did you verify your code works?

• Typecheck: pass (0 errors)
• Lint: pass (0 errors)
• Tests: 2324 pass / 3 fail — failures are pre-existing installation > latest network tests, unrelated to this change

Screenshots / recordings

Checklist

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

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