feat: Add state-based configuration support to MCPToolset

[vblagoje]

Why

Agents need tools that seamlessly share data through state. MCPTool already supports inputs_from_state and outputs_to_state for automatic parameter injection and output handling. MCPToolset couldn't use these features - breaking Agent workflows that rely on state propagation across multiple MCP tools.

This PR completes MCP's Agent integration story. MCPToolset now dynamically configures state mappings for each discovered tool, enabling the same seamless state-based coordination that MCPTool provides.

• fixes Add inputs_from_state and outputs_to_state support to MCPToolset #2677

What

Added per-tool state configuration to MCPToolset:

• Added inputs_from_state, outputs_to_state, outputs_to_string parameters (dict mapping tool names to configs)
• _serialize_state_config() / _deserialize_state_config() helpers handle both nested (outputs_to_state) and flat (outputs_to_string) formats
• _validate_state_configs() warns on unknown tool names
• ValueError from Tool parameter validation bubbles up (Haystack >=2.22.0)
• Updated dependency to haystack-ai>=2.19.0 (supports state params)

How can it be used

from haystack_integrations.tools.mcp import MCPToolset, StdioServerInfo

# Create toolset with per-tool state configuration
toolset = MCPToolset(
    server_info=StdioServerInfo(command="uvx", args=["mcp-server-git"]),
    tool_names=["git_status", "git_diff", "git_log"],
    
    # Map state keys to tool parameters
    inputs_from_state={
        "git_status": {"repository": "repo_path"},
        "git_diff": {"repository": "repo_path"},
        "git_log": {"repository": "repo_path"},
    },
    
    # Map tool outputs to state keys
    outputs_to_state={
        "git_status": {"status_result": {"source": "status"}},
        "git_diff": {"diff_result": {}},
    },
    
    # Convert outputs to strings for LLM
    outputs_to_string={
        "git_diff": {"source": "diff", "handler": format_diff},
    },
)

How did you test it

• 3 unit tests: state config creation, serialization roundtrip, unknown tool warning
• 1 validation test: ValueError on invalid parameter (skipped on Haystack <2.22.0)
• 6 helper tests: serialize/deserialize for both outputs_to_state and outputs_to_string formats

Notes for the reviewer

Parameter validation behavior:

• Haystack >=2.22.0: ValueError raised at tool creation time for invalid parameter names
• Haystack 2.19.0-2.21.x: Invalid parameters fail at runtime during tool invocation

[sjrl]

@vblagoje looking good! I wanted to ask if you could add an integration test using these configs to see if everything works as expected?

[mpangrazzi]

Looks already good! I've added a few comments. +1 on adding an integration test as @sjrl mentioned.

[tstadel]

Nice work. Looks good from my end!

[vblagoje]

This should be gtg now, please verify last three commits @sjrl

• Integration test is similar to our canonical state example but using mcp tools instead of inline tool functions
• Also addressed great comments from @mpangrazzi 🙏

[mpangrazzi]

Looks good!

[sjrl]

Looks good!

[vblagoje]

@tstadel it is ready for you - https://pypi.org/project/mcp-haystack/1.2.0/