feat: add process topology validation (cycle detection, input connectivity, event matching)

[Copilot]

Summary

Implements validation checks in plugboard-schemas for ProcessSpec objects: circular connection detection (with initial_values resolution), input connectivity checking, and input event producer matching. Includes a dependency-free implementation of Johnson's algorithm for cycle finding.

Changes

• plugboard-schemas/plugboard_schemas/_graph.py — Johnson's algorithm for finding all simple cycles in a directed graph, backed by Tarjan's SCC. No additional dependencies.
• plugboard-schemas/plugboard_schemas/_validation.py — Three validation utilities:
  • validate_no_unresolved_cycles — Rejects circular connections unless initial_values is set on a target input within the loop. Integrated as a @model_validator on ProcessSpec.
  • validate_all_inputs_connected — Checks all component inputs have a connector targeting them. Runtime utility (requires component IO info unavailable at schema level).
  • validate_input_events — Checks every component input event type has a corresponding output event producer in the process. Runtime utility.
• plugboard-schemas/plugboard_schemas/process.py — Added _validate_no_unresolved_cycles model validator to ProcessSpec.
• plugboard-schemas/plugboard_schemas/__init__.py — Exports simple_cycles, validate_all_inputs_connected, validate_input_events, validate_no_unresolved_cycles.
• tests/unit/test_process_validation.py — 22 tests covering graph algorithms and all three validators.

Cycle detection runs automatically at schema validation time:

# This raises ValueError — circular connection without initial_values
ProcessSpec.model_validate({
    "args": {
        "components": [
            {"type": "my.CompA", "args": {"name": "a"}},
            {"type": "my.CompB", "args": {"name": "b"}},
        ],
        "connectors": [
            {"source": "a.out", "target": "b.in_1"},
            {"source": "b.out", "target": "a.in_1"},
        ],
    }
})

# This passes — initial_values breaks the cycle
ProcessSpec.model_validate({
    "args": {
        "components": [
            {"type": "my.CompA", "args": {"name": "a", "initial_values": {"in_1": [0]}}},
            {"type": "my.CompB", "args": {"name": "b"}},
        ],
        "connectors": [
            {"source": "a.out", "target": "b.in_1"},
            {"source": "b.out", "target": "a.in_1"},
        ],
    }
})

Warning

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• metadata.google.internal
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Original prompt

---

This section details on the original issue you should resolve

<issue_title>Model validation (valid model topology etc)</issue_title>
<issue_description>Implement validation checks in plugboard-schemas for Process objects. Start with implementing the following checks:

1. Check that all component inputs are connected.
2. Check all components with input events, and make sure there exists in the process a component that outputs the required event type.
3. Check for circular connections within the process topology. These circular loops must only be considered valid if there are initial_values set on an appropriate component input within the loop.

For (3) we can use Johnson's algorithm to find all simple circuits within the process. Implement this as a utility in plugboard-schemas, i.e. try not to introduce additional dependencies. Info on the algorithm can be found at https://github.com/qpwo/python-simple-cycles/blob/master/johnson.py.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Model validation (valid model topology etc) #95

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