feat: auto-detect pip mirror

[BeautyyuYanli]

Description

This PR adds startup-time automatic PyPI mirror selection for plugin dependency installation.

Environment variables

• PIP_MIRROR_AUTO_DETECT

  • Default: true
  • Controls whether the daemon should try to auto-select a suitable PyPI mirror during startup.
  • When enabled and no explicit mirror is configured, the daemon detects the public IP region and may assign a local mirror for regions with poor network connectivity.

• PIP_MIRROR_URL

  • Default: empty
  • Manually specifies the PyPI mirror used for plugin dependency resolution.
  • When set, it takes precedence over auto detection and disables the auto-selected mirror.

Detection behavior

• The daemon uses https://cloudflare.com/cdn-cgi/trace at startup to detect the public IP region.
• If the detected region needs a local mirror, config.PipMirrorUrl is populated before plugin runtimes install Python dependencies.
• Regions that do not need a mirror keep the default behavior.
• Detection failures are non-fatal: the daemon logs a warning and continues startup without changing the mirror configuration.

Documentation and tests

• Updates .env.example and README.md to describe the mirror-related configuration behavior.
• Adds tests for config defaults, env overrides, trace parsing, mirror precedence, region-based application, and non-blocking failure handling.

Type of Change

• Bug fix
• New feature
• Refactor
• Performance improvement
• Other

Essential Checklist

Testing

• I have tested the changes locally and confirmed they work as expected
• I have added unit tests where necessary and they pass successfully

Bug Fix (if applicable)

• I have used GitHub syntax to close the related issue (e.g., Fixes #123 or Closes #123)

Additional Information

Local verification performed:

go build -o /tmp/opencode/dify-plugin-daemon-server cmd/server/main.go
go test ./cmd/server ./internal/types/app

This PR is drafted by gpt-5.4 and gpt-5.5. I'm responsible for all the changes. I have reviewed the code and varified the behavior, while breaks may still exist. Reach me to fix in this case.