[Feature] Add pre-deployment validation (dry-run) for tasks

[yunqiqiliang]

Problem

Users can deploy tasks with incomplete or invalid configurations, leading to runtime failures:

• Missing datasource mappings
• Incompatible table schemas
• Circular dependencies
• Invalid cron expressions

Errors are only caught after deployment (TASK_ERROR), requiring rollback and reconfiguration.

Proposed Solution

Add cz-cli task validate <id> [--dry-run] command that checks:

1. Connectivity: Source/target datasource reachability
2. Schema Compatibility: Column type mapping, primary key existence
3. Mapping Completeness: All required tables mapped, no orphan references
4. Dependency Graph: No circular dependencies, valid execution order
5. Schedule Validity: Cron expression syntax, timezone correctness

Output a structured risk report:

✅ Datasource connectivity: OK
⚠️  Type mismatch: resource.is_leaf (BIT -> BOOLEAN)
❌ Missing mapping: table 'auth_path' not configured

Expected Benefit

• Shift error detection left (before deployment).
• Improve first-time deployment success rate.
• Provide actionable feedback for configuration fixes.

Priority

🔴 P0