Fix double-quoted error messages in KeyError handlers

[DeDuckProject]

Summary

When a tool call fails due to an unknown circuit_id, the error field in the JSON response contains extra quotes:

{"success": false, "error": "\"No circuit found with id 'abc'\"}

Expected:

{"success": false, "error": "No circuit found with id 'abc'"}

Root Cause

All KeyError handlers across the tool modules use str(exc) to serialize the exception:

except KeyError as exc:
    return json.dumps({"success": False, "error": str(exc)})

str(KeyError("message")) returns the repr of its first argument, which adds surrounding quotes to strings. Using exc.args[0] instead returns the raw message.

Affected Files

• src/stim_mcp_server/tools/circuit_management.py
• src/stim_mcp_server/tools/simulation.py
• src/stim_mcp_server/tools/analysis.py
• src/stim_mcp_server/tools/visualization.py

Fix

Replace str(exc) with exc.args[0] in every except KeyError handler, e.g.:

except KeyError as exc:
    return json.dumps({"success": False, "error": exc.args[0]})

Notes

• Pre-existing issue (present in original server.py before modularization)
• Non-blocking: existing tests pass because the circuit_id substring is still present inside the quoted string
• Found during QA of feature/generate-circuit-modularize