feat(mcp): non-text ToolContent kinds (image/audio/resource)

[Svagtlys]

Description

Extend octave.mcp.types.ToolContent beyond kind="text" to cover image, audio, and resource-link content blocks returned by tools/call. Non-text blocks are currently skipped silently in McpClient.call_tool.

Why

Servers like filesystem and vision-oriented MCP servers return images/audio/resources; skipping them drops data callers may need (e.g. tool-produced charts, fetched files). Deferred deliberately in the client-core design until a caller needs it.

Scope

• New ToolContent variants (image: base64 + mimeType; audio; resource link/resource contents) as a discriminated union
• Map the corresponding SDK content types in McpClient.call_tool (keep SDK types quarantined to client.py)
• Tests: harness returns each new block kind; unknown/future kinds stay skipped

References

• Design spec "Open follow-ups to file after this issue": .agents/specs/2026-09-10-mcp-client-core-design.md
• Skipped-block comment: McpClient.call_tool docstring, PR feat(mcp): implement MCP client core (JSON-RPC 2.0) #81