Defer ODE input validation until ProtWise runtime

[bibymaths]

Motivation

• Prevent import-time validation of [ode.inputs] so generic imports, logging, networkmodel imports, and pytest collection don't require ProtWise ODE inputs to be present.
• Preserve strict validation semantics for the actual ProtWise/ODE run so missing inputs still raise clear errors at runtime.

Description

• Replaced import-time required ODE path access in config/constants.py with import-safe defaults by returning an empty string when an ODE input is absent and added validate_ode_inputs() to perform strict checks on demand.
• Added _validate_path_value() helper in config/constants.py to centralize required-key and optional-existence checks and exposed validate_ode_inputs(config, require_existing) for runtime validation.
• Made INPUT_EXCEL_* constants default-safe ("") for generic imports in config/constants.py and updated config/config.py so defaults do not fabricate placeholder input paths when [ode.inputs] is absent.
• Wired runtime validation into protwise/runner/main.py by calling validate_ode_inputs(config) before the pipeline reads input files, so missing inputs produce errors only when running ProtWise.

Testing

• Attempted to run the project task pixi install -e dev and pixi run -e dev coverage-all but pixi is not available in this environment so those could not be executed.
• Ran python -m pytest -q, which progressed past the previous import-time KeyError for [ode.inputs] and instead failed during collection due to missing external test dependencies (ModuleNotFoundError: No module named 'numpy', jax, nbformat, etc.), indicating import-time ODE input validation no longer blocks collection.
• Verified modified Python files parse cleanly with ast.parse to ensure syntax correctness after edits.

Files changed and rationale:

• config/constants.py: make ODE input constants import-safe, add validate_ode_inputs() and path validation helper so required checks are deferred to runtime.
• config/config.py: stop fabricating default ODE input paths when inputs are absent so CLI/runtime reflects missing inputs rather than invented paths.
• protwise/runner/main.py: call validate_ode_inputs(config) prior to reading input files so ProtWise execution still enforces required inputs.

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Codex Task