feat: validate SSH key paths must be absolute

[josecelano]

Fixes #225

Summary

This PR implements validation to ensure SSH key paths are absolute during create environment command, preventing failures later during configure and other commands.

Changes

• Validation Logic: Added is_absolute() checks for both private and public SSH key paths in SshCredentialsConfig::new()
• Error Handling: Added two new error variants:
  • RelativePrivateKeyPath - for relative private key paths
  • RelativePublicKeyPath - for relative public key paths
• Error Messages: Comprehensive help text with:
  • Clear explanation of the problem
  • realpath command examples for fixing
  • Rationale for why absolute paths are required
• Tests: Added 5 new unit tests demonstrating validation behavior
• Test Updates: Updated 23 existing tests to use absolute paths via CARGO_MANIFEST_DIR
• Doc Examples: Marked 6 doc examples as no_run to prevent execution

Validation

✅ All 1429 unit tests passing
✅ All 315 doctests passing
✅ All pre-commit checks passing
✅ Manual E2E testing confirmed:

• Relative private key path → rejected with clear error
• Relative public key path → rejected with clear error
• Absolute paths → accepted and environment created successfully

Example Error Output

❌ Create command failed: Configuration validation failed: SSH private key path must be absolute: "fixtures/testing_rsa"

SSH private key path must be absolute.

Fix:
1. Convert relative path to absolute path:

Use the `realpath` command to get the absolute path:

realpath <your-relative-path>

Example:
- Current (relative): fixtures/testing_rsa
- Command: realpath fixtures/testing_rsa
- Result: /home/user/project/fixtures/testing_rsa

2. Update your configuration file with the absolute path

Benefits

• Fail Fast: Errors caught immediately during config parsing, before any infrastructure operations
• Clear Guidance: Users get actionable error messages with specific fix instructions
• Prevents Later Failures: Eliminates cryptic SSH failures during configure and subsequent commands
• Multi-command Safety: Ensures paths work correctly across different working directories

[josecelano]

ACK 0ded857