feat(sdk-go): implement MapFileParser

[macalbert]

Description

Implement the map file JSON parser that reads $config section and variable mappings from a JSON map file. Returns a ParsedMapFile struct with config and mappings separated.

Acceptance Criteria

• internal/application/parser.go: ParseMapFile(path string) (*ParsedMapFile, error)
• Parses $config section (provider, vaultUrl, profile) into MapFileConfig
• Parses remaining keys as variable mappings (map[string]string)
• Handles missing $config gracefully (defaults to AWS)
• Returns clear error for invalid JSON or missing file
• Unit tests with testify: valid file, missing config, invalid JSON, empty mappings
• go test ./... passes

Technical Notes

• Lives in internal/application/ (not publicly exported)
• ParsedMapFile struct: Config MapFileConfig + Mappings map[string]string
• Must match map-file format used by all other SDKs
• Depends on: feat(sdk-go): add domain types and SecretProvider interface #204 (domain types)