No progressToken forwarding or progress notification bridging for downstream tools

[QuentinBisson]

Problem

muster cannot relay per-request progress from downstream tools:

• The internal MCPClient interface is CallTool(ctx, name, args) (internal/aggregator/types.go:38) — there is no way to thread a progressToken / request _meta to the downstream server.
• notifications/progress from downstream servers is not bridged back to the calling client session.

Long-running downstream tools (cluster provisioning, large queries) give zero feedback through muster; clients see nothing until the final result or a timeout.

Scope

This is a structural change, intentionally split from the serialization fixes:

1. Extend the MCPClient interface (or add a variant) to accept request _meta, and forward the caller's progressToken on call_tool invocations.
2. Bridge notifications/progress from the downstream connection to the calling session, translating the token.
3. Decide behavior for pooled/session connections where one downstream client serves multiple aggregator sessions.

Related but explicitly out of scope: sampling and elicitation passthrough (downstream server-initiated requests). That is a separate design discussion about whether muster should proxy interactive capabilities at all.