feat: Add ROS Service (.srv) schema parser

[zhexuany]

Issue Description

Robocodec currently supports parsing ROS message definitions (.msg files) but does not support ROS service definitions (.srv files). Services are a core ROS communication pattern used for request/response operations.

Background

A ROS service file defines three messages separated by "---":

# Example: SetBool.srv
bool data   # Request
---
bool success   # Response
string message   # Response

Proposed Solution

Extend the schema parser to handle .srv files with a new ServiceSchema type.

Requirements

• Add srv_parser.rs module in src/schema/parser/
• Parse service definition format (request --- response)
• Support ROS1 and ROS2 service formats
• Integrate with unified parser (auto-detect .srv files)
• Add ServiceSchema type to src/schema/ast.rs
• Support service type references in message schemas
• Add unit tests for various service formats
• Add integration tests with real .srv files
• Update documentation

Acceptance Criteria

1. Can parse standard ROS service definitions
2. Correctly separates request and response messages
3. Handles nested types and arrays in service definitions
4. Round-trip parsing preserves all information

Test Cases

• Simple services (primitive types only)
• Services with arrays
• Services with nested message types
• Real-world services (e.g., std_srvs/SetBool, nav_srvs/GetPlan)

References

• Existing msg parser: src/schema/parser/msg_parser/
• ROS Services documentation: https://wiki.ros.org/srv

---

Priority: P1 - Core ROS completeness
Estimated effort: Low-Medium