test(ci): run all tests by default

[alexey-igrychev]

Summary

All tests now follow one convention regardless of author: ordinary *_test.go files, Test… names, and default test tasks. The server CI job runs the previously excluded tests and contributes their coverage.

What

• task server:test:unit compiles and runs every server test; CI starts buildkitd and supplies TRDL_SMOKE_BUILDKITD_ADDRESS to run its smoke test.
• task e2e:test:e2e includes the mac-signing suite; its Docker, Git, GPG, and git-signatures requirements remain unchanged.
• Only the live-BuildKit smoke test is gated, by its TRDL_SMOKE_BUILDKITD_ADDRESS requirement rather than test authorship.
• AGENTS and review guidance no longer require AI-specific filenames, tags, function prefixes, or helpers.
• No end-user CLI or release behavior changes.

Why

The ai_tests build tag omitted those tests from normal task runs, CI, and coverage, allowing tests to rot unnoticed. Keeping a second task and CI job would preserve the same authorship-based split, so the migration folds their environment setup into the standard server job instead.

Fixes #413

[alexey-igrychev]

Verification

• CI Lint workflow passed after 338f9fa and fa835df corrected gofumpt and misspell violations exposed by the formerly excluded files.
• CI Tests workflow passed: server unit tests, default E2E tests, BuildKit-client E2E, and configured-buildx E2E.
• task server:test:unit paths='./ ./pkg/docker ./pkg/pgp' passed locally.
• Mutation: invert ValidateBuildxDriver allow-list → moved TestValidateBuildxDriver and related builder tests failed; restored implementation.

Review focus

• Confirm the consolidated unit_server job retains the BuildKit daemon and smoke-test address while removing only the authorship-based test split.