Add build environment configuration and validation

[JAORMX]

Summary

Add configuration model and validation for build-time environment variables that will be injected into protocol build Dockerfiles (npx://, uvx://, go://). This enables organizations to configure custom package mirrors for corporate environments.

• Add BuildEnv map to Config struct for storing environment variables
• Add validation functions for env var keys (must be uppercase, no reserved system keys like PATH, HOME)
• Add validation functions for env var values (reject shell metacharacters to prevent injection)
• Extend Provider interface with build env operations (Set, Get, GetAll, Unset, UnsetAll)
• Implement for DefaultProvider, PathProvider, and KubernetesProvider (no-op)
• Add comprehensive tests for validation and provider operations

This is the foundation for THV-2732 custom package registry support. The CLI commands and Dockerfile template integration will follow in a separate PR.

Test plan

• Unit tests for key validation (format, reserved keys)
• Unit tests for value validation (dangerous characters)
• Unit tests for provider operations (PathProvider, KubernetesProvider)
• task test passes
• task lint passes

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 88.63636% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 55.83%. Comparing base (1200343) to head (54884a3).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/config/interface.go	66.66%	10 Missing ⚠️

Additional details and impacted files

@@ Coverage Diff @@ ## main #2740 +/- ## ========================================== + Coverage 55.73% 55.83% +0.10%  ========================================== Files 314 315 +1 Lines 30510 30598 +88 ========================================== + Hits 17004 17084 +80  - Misses 12019 12028 +9  + Partials 1487 1486 -1 

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.