Allow parent request handler to initialize context variables / state / env

[brettimus]

Sometimes you have information that you want to pass down to tools via context.

Currently, there is no way for a parent request to initialize context for the server, like this:

app.all("/mcp", async (ctx) => {
    const db = ctx.get("db");
    const response = await httpHandler(ctx.req.raw, { state: { db } });
    return response;
})

Such that tools have access to an underlying database connection:

mcp.tool("my-tool", {
    handler: (args, ctx) => {
        // ... do something with ctx.state.db
    },
})

We will need to employ some optimistic typing tricks similar to Hono to make this happen.

[brettimus]

realistic example: say your server is in a durable object, and you want to give tools access to the DO instance's sqlite database.

you'd either have to define the MCP server tools inside the DO constructor, or create a factory function that accepts the sqlite connection and pass it downstream to all tool handlers (DI-like pattern)

if we want Hono ergonomics, we need to be able to pass down context