feat(mcp-chat): added shared libraires and extension points

[fjudith]

Hey, I just made a Pull Request!

Starts the Implementation of #7976 following the adr011 and superseeds #8196.

• add the isomorphic package for shared types and future support for translations and permissions
• add the node library to share the extension point between the backend and backend module

flowchart LR
    subgraph sg1["Common Library: plugin-mcp-chat-common"]
        TYPES["Shared Types (ChatMessage, Tool, etc.)"]
    end

    subgraph sg2["Node Library: plugin-mcp-chat-node"]
        EP["llmProviderExtensionPoint"]
        BASE["LLMProvider (abstract), ProviderConfig"]
    end

    subgraph sg3["Core Plugin: plugin-mcp-chat-backend"]
        SVC["MCPClientServiceImpl"]
        ROUTER["createRouter"]
        PLUGIN["mcpChatPlugin (createBackendPlugin)"]
        MOD_OAI["module-openai"]
        MOD_ANT["module-anthropic"]
        MOD_GEM["module-gemini"]
        MOD_OLL["module-ollama"]
        MOD_LIT["module-litellm"]
        MOD_OAR["module-openai-responses"]
        PF["provider-factory"]
    end

    EP -->|depends on| BASE
    PLUGIN -->|depends on| PF
    PLUGIN -->|depends on| TYPES
    SVC -->|uses| BASE

    PF -->MOD_OAI
    PF -->MOD_ANT
    PF -->MOD_GEM
    PF -->MOD_OLL
    PF -->MOD_LIT
    PF -->MOD_OAR
    PF -->BASE

    PLUGIN -->|reads registry from| EP
    PLUGIN -->|resolves active provider, injects into| SVC

    MOD_OAI -->|extends| BASE
    MOD_ANT -->|extends| BASE
    MOD_GEM -->|extends| BASE
    MOD_OLL -->|extends| BASE
    MOD_LIT -->|extends| BASE
    MOD_OAR -->|extends| BASE

    MOD_OAI -->|extends| TYPES
    MOD_ANT -->|extends| TYPES
    MOD_GEM -->|extends| TYPES
    MOD_OLL -->|extends| TYPES
    MOD_LIT -->|extends| TYPES
    MOD_OAR -->|extends| TYPES

    MOD_GEM -->|depends on| GOOGLE["@google/genai"]
    MOD_OLL -->|depends on| OLLAMA["ollama"]

    style sg1 fill: lightgreen
    style sg2 fill: lightgreen
    style PF fill: #FDD835

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✔️ Checklist

• A changeset describing the change and affected packages. (more info)
• Added or updated documentation
• Tests for new functionality and regression tests for bug fixes
• Screenshots attached (for UI changes)
• All your commits have a Signed-off-by line in the message. (more info)

[backstage-goalie]

Changed Packages

Package Name	Package Path	Changeset Bump	Current Version
@backstage-community/plugin-mcp-chat-backend	workspaces/mcp-chat/plugins/mcp-chat-backend	minor	v0.8.0
@backstage-community/plugin-mcp-chat-common	workspaces/mcp-chat/plugins/mcp-chat-common	minor	v0.1.0
@backstage-community/plugin-mcp-chat-node	workspaces/mcp-chat/plugins/mcp-chat-node	minor	v0.1.0
@backstage-community/plugin-mcp-chat	workspaces/mcp-chat/plugins/mcp-chat	minor	v0.6.0