reject file path input defaults

[sarptandoven]

summary

• reject non-None defaults for Path and deprecated File inputs during static schema parsing
• keep default=None working for optional file/path inputs
• track unresolved Input(default=...) expressions so Path("...") defaults no longer silently become None
• add regression coverage for string, constructor, and list defaults

why

• closes Disallow setting default for files/paths #606
• generated schemas cannot reliably represent file/path defaults today
• silently accepting these defaults can make the schema claim a usable default that downstream consumers cannot provide

validation

• go test ./pkg/schema/python -run 'TestPathAndFileInputsRejectDefaults|TestOptionalInputPipeNone|TestMultipleInputsWithDefaults' -count=1
• go test ./pkg/schema/... -count=1
• go test ./pkg/config ./pkg/util -count=1
• git diff --check

note

• go test ./pkg/... -count=1 was also attempted. The schema/config/util packages passed, but Docker-dependent packages failed because this machine cannot connect to a Docker daemon at unix:///var/run/docker.sock / rootless Docker is unavailable.