fix: config violations when running db migtration ci tests

[bowenliang123]

Checklist:

Important

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• Please open an issue before creating a PR or link to an existing issue
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

Description

• fix dify_config violation when running db migration tests with .env.example for config values, caused by missing default values in .env.example which is introduced in feat: The SSRF request timeout configuration item is added #10292
  log: https://github.com/langgenius/dify/actions/runs/11727349457/job/32668206690#step:9:42

pydantic_core._pydantic_core.ValidationError: 4 validation errors for DifyConfig
SSRF_DEFAULT_TIME_OUT
  Input should be a valid number, unable to parse string as a number [type=float_parsing, input_value='', input_type=str]
    For further information visit https://errors.pydantic.dev/2.9/v/float_parsing
SSRF_DEFAULT_CONNECT_TIME_OUT
  Input should be a valid number, unable to parse string as a number [type=float_parsing, input_value='', input_type=str]
    For further information visit https://errors.pydantic.dev/2.9/v/float_parsing
SSRF_DEFAULT_READ_TIME_OUT
  Input should be a valid number, unable to parse string as a number [type=float_parsing, input_value='', input_type=str]
    For further information visit https://errors.pydantic.dev/2.9/v/float_parsing
SSRF_DEFAULT_WRITE_TIME_OUT
  Input should be a valid number, unable to parse string as a number [type=float_parsing, input_value='', input_type=str]
    For further information visit https://errors.pydantic.dev/2.9/v/float_parsing

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update, included: Dify Document
• Improvement, including but not limited to code refactoring, performance optimization, and UI/UX improvement
• Dependency upgrade

Testing Instructions

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Test A
• Test B