feat(mcp): inbound request handler registry (sampling/roots/elicitation)

[Svagtlys]

Description

Handle inbound server→client JSON-RPC requests (sampling, roots, elicitation) via a pluggable handler registry on McpClient. Today the SDK answers these itself (unset callbacks → JSON-RPC error response); the seam is pinned by test_inbound_request_rejected_with_method_not_found.

Why

MCP servers use sampling/createMessage to ask the client to run inference, roots/list to scope file access, and elicitation to request user input. Octave's agent flows (prompt assembly, permission scoping) need to answer these instead of erroring.

Scope

• Registry API on McpClient (e.g. on_sampling(...)/on_roots(...) callbacks wired into ClientSession kwargs at connect time)
• Default behavior unchanged: no handler → SDK error response (never silence)
• Note: SDK 1.30 validates inbound requests against the typed union and answers sampling/createMessage with -32600 via the unset-callback path, not -32601 (see PR feat(mcp): implement MCP client core (JSON-RPC 2.0) #81)
• Tests over the in-process harness (tests/mcp/conftest.py Peer.send_request)

References

• Design spec decision 4 + "Open follow-ups": .agents/specs/2026-09-10-mcp-client-core-design.md
• Seam pinned in: octave.mcp.client _on_inbound_message docstring, PR feat(mcp): implement MCP client core (JSON-RPC 2.0) #81