Update Dagger to 0.21.9 and test dependencies

[gauravtiwari]

What changed?

• Update the exact supported Dagger CLI, engine image, documentation, tests, and live CI job from 0.21.8 to 0.21.9.
• Update SimpleCov from 1.1.1 to 1.2.0 and WebMock from 3.26.3 to 3.26.4.
• Update the lockfile metadata to Bundler 4.0.20, which supports the project minimum of Ruby 3.2.
• Keep the runtime json ~> 2.21 constraint and json 2.21.2 lock unchanged.
• Make ProgressRunner tests control the startup-notice delay so host scheduling cannot make fast subprocess fixtures fail. The production one-second delay is unchanged.

Dagger 0.21.9 adds the SSHFS volume API and fixes directory state, session lifecycle, cache metadata, registry progress, and registry-auth cache reuse. Its schema changes are additive for the API covered by this client. The existing schema contract passed unchanged against the 0.21.9 engine.

Verification

• Ruby 4.0.6: bundle _4.0.20_ exec bin/ci
• Ruby 3.2.11: bundle _4.0.20_ exec bin/ci
• Dagger 0.21.9 live suite: 7 runs, 464 assertions, no failures or errors
• Packaged gem file list and runtime dependencies inspected
• No credentials, personal data, generated gems, or dependency directories