feat(config): add schema-based configuration validator with field error reporting (#694)

[ravendevhub]

Summary

Resolves #694 by implementing a deterministic, schema-based configuration validation engine (listener/src/config/schema-validator.ts) that enforces explicit types, bounds, and enums on environment configurations while aggregating violations and identifying the exact affected field.

Changes

1. Schema Validation Engine (listener/src/config/schema-validator.ts):
   • Validates required fields (CONTRACT_ADDRESSES), network enums (STELLAR_NETWORK), log levels (LOG_LEVEL), and numeric ranges (POLL_INTERVAL_MS, PORT).
   • Custom ConfigSchemaValidationError pinpoints exact offending field paths and reasons.
2. Automated Unit Tests (listener/src/config/schema-validator.test.ts):
   • Tests valid configurations, missing required fields, enum validation, and numeric out-of-bounds errors.
3. Documentation (docs/CONFIGURATION_SCHEMA_VALIDATION.md):
   • Documents configuration schema matrix and validation error formats.

Acceptance Criteria

• Configuration fields have explicit types.
• Required values are validated.
• Numeric and enumerated values are validated.
• Validation errors identify the affected configuration field.