feat: add Dagger lint and test pipelines

[MChorfa]

Summary

Adds comprehensive lint and test pipelines using Dagger.

Changes

• dagger/pipelines/lint.go: Go build linting
• dagger/pipelines/test.go: Full test suite with mock server
• dagger/pipelines/test.go: Fast unit tests (TestFast)
• dagger/main.go: Refactored to use pipelines package

Features

• Lint: Runs go build ./...
• Test: Full tests with mock server service
• TestFast: Short unit tests only

Dependencies

PR 7 (Dagger Bootstrap)

Risk

Medium - builds on Dagger infrastructure.

[MChorfa]

Closing this PR due to merge conflicts with upstream CI/CD workflow files.

The Dagger module conflicts with:

• .github/workflows/ci.yml (upstream has their own CI)
• dagger/ directory additions may overlap with upstream changes

Since upstream uses Stainless for SDK generation and has established CI, adding Dagger would require coordination with their workflow. The remaining rebased PRs (#5, #6, #7) are smaller focused improvements that don't conflict with CI infrastructure.