fix: require-runtime-dependencies not installed in build environment

[psyclaudeZ]

Summary

Addressing #2110.

Mentioned in the issue, #2073 changed prepare_build_environment such that:

1. prepare_environment syncs the build env, caching dependencies_complex, all_dependencies_complex, and missing_dependencies via @cached_property.
2. Queries hatchling for additional runtime deps and appends them to additional_dependencies
3. Calls sync_dependencies again but the cached properties from step 1 still have empty additional_dependencies so runtime deps are never installed.

This PR replaces @cached_property with @property for those fields. Alternatively, we could manually evict the fields after mutating additional_dependencies. However, I personally feel the latter is too ad-hoc and bypasses a more serious issue (caching properties that depend on mutable state).

Test Plan

Disclaimer: I used Claude Code for these. TDD:

• Added a new integration test.
• Repro steps in the issue.