Fix Value serialization and implement is_empty() method

[briansrls]

Summary

This PR addresses three critical issues in the Value type system and code generation:

1. Implement Value::is_empty() method - Adds semantic emptiness checking across all Value variants
2. Fix silent degradation in value serialization - Replace catch-all <MOCK> placeholders with explicit panics to surface real bugs
3. Improve NonEmpty matcher code generation - Use the new is_empty() method instead of fragile string-only checks

Key Changes

Core IR (core/ir/src/value.rs)

• Added Value::is_empty() method with variant-specific semantics:
  • Unit, Skipped → always empty
  • Str, List, Map, Secret → check inner emptiness
  • Bool, Int, Json, Request, Response → never empty (data by existence)

Code Generation (core/codegen/src/testgen/codegen.rs)

• value_to_rust_literal():
  • Fixed string escaping to handle backslashes before quotes
  • Replaced filter_map on List that silently dropped non-string elements with proper recursive serialization
  • Added full Value::Map support with proper BTreeMap construction
  • Changed catch-all _ => "<MOCK>" to explicit panic for Request/Response variants
  • Added documentation explaining the intentional panic behavior

Test Mocking (core/test/src/mock_spec.rs)

• value_to_code(): Applied same fixes as value_to_rust_literal() for consistency
• OutputMatcher::NonEmpty: Simplified assertion from as_str().map(|s| s.is_empty()).unwrap_or(false) to is_empty(), making it type-aware and supporting all Value variants

Generated Tests

• Updated all generated test files to use the new is_empty() assertion format
• Regenerated with updated content hashes

Implementation Details

The panic-on-serialization approach is intentional. Previously, Request and Response variants would silently degrade to <MOCK> strings, hiding real bugs in test generation. Now they fail loudly, forcing explicit handling or use of Value::Skipped as a placeholder.

String escaping now properly handles both backslashes and quotes, preventing malformed Rust code generation.

Closes

• Hack 3: Make NonEmpty codegen type-aware
• Hack 4: Replace value_to_rust_literal catch-all with panic
• Hack 4: Fix Value::List filter_map silent dropping
• Task: Add Value::is_empty() method to core/ir

https://claude.ai/code/session_015sAGUqUgBu22chUq2qftdq