Modularize API MCP and domain tests

[yvette-carlisle]

Summary

• Split API route/auth/request-validation test modules into focused Rust modules.
• Split MCP server schema and tool-definition tests into focused modules with explicit helper visibility.
• Split elf-domain writegate and memory-policy support tests into focused modules, including integration-test sibling modules for #[path] layout.

Batch size

• 46 files changed
• 2463 insertions, 2336 deletions
• 4799 total changed lines

Validation

• After MCP segment: RUSTC=/run/current-system/sw/bin/rustc cargo make fmt-rust && RUSTC=/run/current-system/sw/bin/rustc cargo make check
• After API segment: RUSTC=/run/current-system/sw/bin/rustc cargo make fmt-rust && RUSTC=/run/current-system/sw/bin/rustc cargo make check
• After domain segment/final: RUSTC=/run/current-system/sw/bin/rustc cargo make fmt-rust && RUSTC=/run/current-system/sw/bin/rustc cargo make check
• Final result: 403 tests run: 403 passed, 92 skipped
• git diff --check