Implement domain configuration layer for environment creation

[Copilot]

Adds configuration infrastructure in the domain layer to bridge external configuration sources (JSON, CLI) and the internal domain model for environment creation.

Changes

Configuration Value Objects

• EnvironmentCreationConfig - Top-level config with to_environment_params() converting to Environment::new() parameters
• SshCredentialsConfig - Config layer representation (distinct from adapters::ssh::SshCredentials), supports defaults (username: "torrust", port: 22)
• EnvironmentSection - Environment-specific settings container

Type Conversion

• String → EnvironmentName (validates lowercase, dashes, no leading numbers)
• String → Username (validates Linux requirements, 1-32 chars)
• String → PathBuf (validates file existence for SSH keys)

Error Handling

• CreateConfigError enum with variants: InvalidEnvironmentName, InvalidUsername, PrivateKeyNotFound, PublicKeyNotFound, InvalidPort
• All variants implement .help() providing actionable troubleshooting guidance

Module Structure

src/domain/config/
├── mod.rs                          # Module exports and docs
├── environment_config.rs           # EnvironmentCreationConfig
├── ssh_credentials_config.rs       # SshCredentialsConfig
└── errors.rs                       # CreateConfigError

Usage

use torrust_tracker_deployer_lib::domain::config::EnvironmentCreationConfig;
use torrust_tracker_deployer_lib::domain::Environment;

let json = r#"{
    "environment": { "name": "dev" },
    "ssh_credentials": {
        "private_key_path": "fixtures/testing_rsa",
        "public_key_path": "fixtures/testing_rsa.pub"
    }
}"#;

let config: EnvironmentCreationConfig = serde_json::from_str(json)?;
let (name, credentials, port) = config.to_environment_params()?;
let environment = Environment::new(name, credentials, port);

Test Coverage

26 unit tests covering deserialization, type conversion, validation, error handling, and integration with Environment::new().

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Part of Epic #34 (Create Environment Command). Enables application command handlers (#36), CLI integration (#37), and E2E tests (#38).

Original prompt

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This section details on the original issue you should resolve

<issue_title>[Subissue 1/7] Core Configuration Infrastructure</issue_title>
<issue_description>## Overview

Implement core configuration infrastructure in the domain layer with pure configuration value objects, business validation rules, and explicit error handling.

Parent EPIC: #34 - Implement Create Environment Command
Related: Full Specification | Roadmap Task 1.5

Key Architecture Points

• Configuration uses SshCredentialsConfig (distinct from adapters::ssh::SshCredentials)
• Converts config strings to domain types (Username, PathBuf)
• JSON Schema validation is optional - can defer to focus on domain validation first
• All errors implement .help() methods (tiered help system pattern)

Goals

• Create configuration value objects that convert to existing domain types

  • EnvironmentCreationConfig with to_environment_params() method
  • SshCredentialsConfig (config layer) distinct from adapters::ssh::SshCredentials
  • Convert string username to shared::Username type
  • Convert string paths to PathBuf
  • Use existing EnvironmentName::new() validation

• Configuration validation

  • Primary validation via domain layer (EnvironmentName, Username, file existence)
  • Follow existing error handling patterns with thiserror and .help() methods
  • Optional: Secondary validation via JSON Schema

• Add comprehensive unit tests

  • Test conversion to Environment::new() parameters
  • Test Username type conversion from string
  • Test compatibility with repository.save(&environment.into_any())
  • Test invalid username and environment name handling

Module Structure

src/domain/config/
├── mod.rs                    # Module exports
├── environment_config.rs     # EnvironmentCreationConfig value object
├── ssh_credentials_config.rs # SshCredentialsConfig value object
├── errors.rs                 # ConfigValidationError enum with .help()
├── schema.rs                 # Optional: JSON Schema validation
└── validation.rs             # Optional: Domain validation helpers

Key Types

EnvironmentCreationConfig

pub struct EnvironmentCreationConfig {
    pub environment: EnvironmentSection,
    pub ssh_credentials: SshCredentialsConfig,
}

impl EnvironmentCreationConfig {
    pub fn to_environment_params(self) 
        -> Result<(EnvironmentName, SshCredentials, u16), CreateConfigError> {
        // Convert to existing domain types
    }
}

SshCredentialsConfig

pub struct SshCredentialsConfig {
    pub private_key_path: String,
    pub public_key_path: String,
    #[serde(default = "default_ssh_username")]
    pub username: String,
    #[serde(default = "default_ssh_port")]
    pub port: u16,
}

Error Types with .help()

#[derive(Debug, Error)]
pub enum CreateConfigError {
    #[error("Invalid environment name")]
    InvalidEnvironmentName(#[source] EnvironmentNameError),
    
    #[error("Invalid SSH username")]
    InvalidUsername(#[source] UsernameError),
    
    // ... other variants
}

impl CreateConfigError {
    pub fn help(&self) -> &'static str {
        // Detailed troubleshooting guidance
    }
}

Acceptance Criteria

• EnvironmentCreationConfig deserializes from JSON
• to_environment_params() converts to domain types successfully
• Invalid environment names rejected with clear errors
• Invalid usernames rejected with clear errors
• SSH key file validation (existence checks)
• All errors implement .help() with detailed guidance
• Unit tests cover all conversion and validation scenarios
• Integration with existing Environment::new() verified

Dependencies

[dependencies]
serde = { version = "1.0", features = ["derive"] }
thiserror = "1.0"

# Optional (can defer):
jsonschema = "0.21"  # JSON Schema validation
serde_json = "1.0"   # JSON Schema handling

Estimated Time

2-3 hours

Notes

• JSON Schema validation is optional enhancement - domain validation is primary
• Focus on type-safe conversion from config strings to domain types
• Maintain clean separation: config layer vs domain/adapters layer
• All error messages must be actionable with .help() methods</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [Subissue 1/7] Core Configuration Infrastructure #35

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[josecelano]

ACK fea50f4