feat: adds input and output validation

[claymcleod]

Added nested validation blocks to task and workflow input and output sections so authors could express boundary conditions without running a helper task. Input validation aggregated failures before execution, while output validation checked completed values before exposing them. Closes #657.

Restricted input validation dependencies to input declarations to preserve DAG construction, required comma-separated clauses for unambiguous parsing, and reserved validation in WDL 1.4. Input failures were made non-retryable; task output failures followed max_retries because they occur after command execution.

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