Enhance test coverage across entire codebase for codecov.

[MrKeiKun]

Description
This PR enhances test coverage across the tenv codebase for #388. It has now been rebased onto the latest upstream/main and extended with additional coverage.

Recent updates

• Added test coverage for the configurable lock path feature: missing-lock-dir handling, folder-scoped locks, and TENV_LOCK_PATH env var resolution (pkg/lockfile, config, versionmanager)
• Added unit tests for packages that previously had none: versionmanager/semantic/parser/terragrunt, versionmanager/retriever/tofu/dl, versionmanager/retriever/atmos, versionmanager/retriever/terraform, versionmanager/retriever/terragrunt
• Fixed a .gitignore bug: unanchored entries (terraform, terragrunt, atmos, tofu, tenv, terramate) were matching anywhere in the tree, silently hiding new files under source directories with the same names (e.g. versionmanager/retriever/terraform/)

Changes Made

• New test files for all major packages including:
  • Core command packages (cmd/terramate/, cmd/tf/)
  • Configuration management (config/)
  • Package utilities (pkg/)
  • Version management system (versionmanager/)
• Updated dependencies in go.mod and go.sum to support enhanced testing

Test Coverage Areas

• API message handling (pkg/apimsg/)
• Architecture-specific code (pkg/archname/)
• Security and verification (pkg/check/cosign/)
• Download and file operations (pkg/download/, pkg/fileperm/)
• GitHub integration (pkg/github/)
• Compression utilities (pkg/uncompress/)
• Version management (builder, proxy, semantic versioning)
• Library examples and edge cases
• Configurable lock path (pkg/lockfile/, config/)
• Terragrunt HCL/JSON version-constraint parsing (versionmanager/semantic/parser/terragrunt/)
• Retriever asset-name and release-extraction logic (versionmanager/retriever/{atmos,terraform,terragrunt,tofu/dl}/)

Referenced Issue: #388

[MrKeiKun]

i think,i can say this is partial one. i wanna see pipeline though

[MrKeiKun]

Right now, total coverage is at least: 42.7%.

[kvendingoldo]

Checking your PR. I can take some time :)

[MrKeiKun]

@kvendingoldo this looks ready