Fix Gradio Playground MCP argument parsing

[lakshayxi]

Fixes #1080.

Summary

• Parse JSON string arguments for MCP call_tool actions at the web preprocessing boundary.
• Preserve strict deserialization for normal typed payloads.
• Keep invalid, non-object, and malformed JSON subject to existing Pydantic validation.
• Add regression coverage for both serialization behavior and the /web/step path.
• Remove the stale Gradio MCP workaround from AGENTS.md.

Tests

• PYTHONPATH=src:envs uv run pytest tests/core/ -q
• PYTHONPATH=src:envs uv run pytest tests/ -q
• uv run usort check src/openenv/core/env_server/serialization.py tests/core/test_mcp/test_mcp_types.py tests/core/test_web_interface_mcp.py
• uv run ruff format src/openenv/core/env_server/serialization.py tests/core/test_mcp/test_mcp_types.py tests/core/test_web_interface_mcp.py --check
• uv run ruff check src/openenv/core/env_server/serialization.py tests/core/test_mcp/test_mcp_types.py tests/core/test_web_interface_mcp.py

Full suite: 1641 passed, 129 skipped.

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Note

Low Risk
Scoped to the web preprocessing deserialization path with clear validation boundaries and broad test coverage; no auth or data-model changes.

Overview
Fixes Gradio Playground and POST /web/step failures when MCP call_tool actions send arguments as a JSON string (textbox) instead of an object.

deserialize_action_with_preprocessing now JSON-decodes string arguments for type: call_tool into a dict on a copy of the payload (input is not mutated). deserialize_action stays strict—string arguments still fail validation. Malformed JSON, arrays, and scalar JSON strings still raise ValidationError.

Regression tests cover serialization edge cases and an integration test on /web/step. AGENTS.md drops the old workaround note about the broken Playground Step form.

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