Improve test infrastructure for extension types

[kadinrabo]

Two test framework limitations were identified while adding unsigned integer extension
types:

1. Single include per test file — the framework only supports one
   SUBSTRAIT_INCLUDE per .test file. Lifting this would allow more flexible test
   organization.

2. Dynamic extension type discovery — the test coverage infrastructure should
   dynamically discover and load extension types from YAML files rather than requiring
   manual updates when new types are added.

Context:

• feat(extensions): add unsigned integer extension types (u8, u16, u32, u64) #953 (comment)
• feat(extensions): add unsigned integer extension types (u8, u16, u32, u64) #953 (comment)