feat: add Modica Group SMS provider support

[superstructor]

Summary

• Add ModicaSMS provider implementing controller.SMSer interface
• Support provider selection via AUTH_SMS_PROVIDER environment variable
• Maintain backward compatibility with existing Twilio configuration
• Add extensive test coverage including mock server and validation tests

Configuration

New environment variables:

• AUTH_SMS_MODICA_USERNAME - Modica API username
• AUTH_SMS_MODICA_PASSWORD - Modica API password

Provider selection:

• Set AUTH_SMS_PROVIDER=modica to use Modica Group
• Defaults to twilio for backward compatibility

Technical Details

• API Integration: Uses Modica REST v2 API with HTTPS and Basic Authentication
• Input Validation: Enforces international phone format (+ prefix required)
• Error Handling: Parses structured API error responses with fallback for malformed responses
• HTTP Client: Production configuration with 30s timeout, connection pooling, and TLS settings
• Testing: Comprehensive test suite with mock server, validation tests, and integration test support

Test Coverage

• Unit tests for provider initialization and SMS sending
• Mock server tests for various API response scenarios
• Input validation tests for phone format and message content
• Integration tests (requires environment variables)
• Error handling tests for API failures and malformed responses

Before submitting this PR:

Checklist

• No breaking changes
• Tests pass
• New features have new tests
• Documentation is updated

Breaking changes

No breaking changes. The implementation maintains full backward compatibility with existing Twilio configurations and defaults to Twilio when no
provider is specified.

Tests

• All existing tests pass with go test -v ./...
• New comprehensive test suite for Modica provider with 100% coverage
• Mock server tests validate API request/response format
• Integration tests support real API testing with credentials
• Input validation tests ensure proper error handling

Documentation

• Updated environment-variables.md with new Modica configuration options
• Updated AUTH_SMS_PROVIDER documentation to show both twilio and modica options
• Added documentation for AUTH_SMS_MODICA_USERNAME and AUTH_SMS_MODICA_PASSWORD
• Code follows existing patterns and conventions from CLAUDE.md guidelines (as DEVELOPER.md seemed outdated)

Test Plan

• Verify Twilio SMS still works (backward compatibility)
• Test Modica SMS with valid credentials
• Confirm proper error messages for invalid phone formats
• Validate provider switching via environment variable
• Run full test suite: go test -v ./go/notifications/sms/

Warning

Despite extensive tests I havn't actually done e2e testing in the nhost stack with this setup and would appreciate your help with doing so, I can provide test Modica group credentials on DM if needed.

[dbarrosop]

Closing/Reopening in order to re-trigger tests with safe_to_test label.

[dbarrosop]

This looks go to me, the only thing I am unsure about is whether to generate a client automatically from their spec using oapi-codegen or just leave it as you implemented it given how small the surface is. We can probably leave as is for now.

The linter isn't happy, just run:

golines -w  --base-formatter=gofumpt .

To fix the formatting and then golangci-lint run ./....

Re e2e tests, how do you want to do it? Using the cli or the cloud? In the cli case I can build a binary with support for this in the toml, in the cloud case I will need to set the env vars for you.

[superstructor]

@dbarrosop thanks for reviewing and your patience.

Have fixed the lint issues. Happy to provide secrets to you for e2e tests via Discord DM.

[superstructor]

@dbarrosop sorry I think I broke my fork, so I'll re-create this PR with the latest changes as this appears unlinked/unmergable.