fix(go): add Go MCP PR explainer example and safeguard Gemini

[huangjeff5]

Adding a Go example with official Github MCP server. This demo shows that you can easily build an agent that leverages domain-specific tools through an MCP server.

Also fixed Gemini plugin issues along the way:

1. MCP server returns tool input JSON schema where minItems, maxItems, minimum, maximum => are already numbers, but gemini plugin expects strings in the JSON schema and casts to int64/float64. Current plugin implementation is brittle and panics, we handle all possible cases explicitly and use the most precise type.
2. Related issue was happening to "required", "propertyOrdering", "enum" fields. gemini plugin expects []any, but they are actually typed as []string. Handle both cases explicitly.

[huangjeff5]

Removed nil panic handling; handled in https://github.com/firebase/genkit/pull/3512/files#diff-096e7aec5bf004683d663cc97ae7d7794730aa60f225c3c5d2e4519ae38a81e2