fix(variables): accept JSON string and empty input for json_schema

[shiyuanfang2nd]

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Related Issue

Closes #164

Summary

VariableEntity.json_schema previously only accepted dict | None. However:

• The frontend code editor persists the schema as raw JSON text, so the
  backend receives a string and validation fails.
• "Clear schema" on the frontend sends "" instead of dropping the field,
  which also fails validation even though the user intent is "no schema".

Changes

• validate_json_schema is switched to mode="before" so it can pre-process
  the raw input.
• Accept a JSON string and parse it into a dict; raise a clear ValueError
  when the string is not valid JSON.
• Treat None and "" uniformly as "no schema" and return None.
• Reject non-dict / non-string inputs with an explicit error message.
• Existing Draft7Validator.check_schema(...) semantic validation is kept.

Tests

Adds tests/variables/test_input_entities.py covering:

• valid dict input
• valid JSON string input (mirrors the frontend payload)
• None input → None
• "" input → None
• malformed JSON string → ValidationError
• non-object / non-string input (e.g. 123) → ValidationError
• semantically invalid schema, both as dict and as string → ValidationError

No public API or behavior is broken for callers that were already passing
dict | None.

Checklist

• This pull request links the issue it resolves or advances
• This pull request title follows Conventional Commits, and any breaking change is marked with !
• If CLA Assistant prompted me, I signed CLA.md in the pull request conversation

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[shiyuanfang2nd]

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[WH-2099]

Reviewed locally; no blocking issues found.