fix(java): Add validation layer to PromptMetadata.fromConfig()

[nickita-khylkouski]

Summary

• Adds type validation layer to PromptMetadata.fromConfig() before performing unchecked casts
• Provides detailed error messages showing field name, expected type, actual type, and value
• Adds 42 test cases covering all validation scenarios

Problem

The fromConfig() method had 8+ unsafe casts that could cause ClassCastException at runtime when YAML frontmatter contained incorrectly typed values. For example, if tools: "not-a-list" was in YAML, the code would crash with a generic ClassCastException.

Solution

Added validation methods that:

1. Validate all field types using instanceof checks before casting
2. Throw IllegalArgumentException with detailed error messages like:

   Field 'tools' has incorrect type. Expected: List, Actual: String, Value: not-a-list
   

3. Handle nested structures (input, output) with appropriate validation

Test Coverage

Created PromptMetadataTest.java with 42 tests:

• 12 happy path tests (valid configurations)
• 15+ type validation error tests (all fields)
• 10+ nested validation tests (input/output configs)
• 8+ edge case tests (null, empty, complex structures)

Files Changed

• PromptMetadata.java: Added validateConfigTypes() and validateFieldIfPresent() methods
• PromptMetadataTest.java: New comprehensive test file
• BUILD.bazel: Added test target

🤖 Generated with Claude Code

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