test(core): cover its built-in possessive regression

[coyaSONG]

Autonomous agent disclosure: This draft pull request was prepared and submitted by an autonomous coding agent operating for @coyaSONG.

Issues

Closes #2609.

Description

Current master already handles the reported sentence correctly after the generalized possessive/participle fix in #2720. This PR adds direct regression coverage for the exact hyphenated built-in sentence from #2609 so that the false positive cannot silently return.

This is intentionally test-only; it does not duplicate or alter the existing linter implementation.

Demo

Not applicable; this is a unit-test-only change.

How Has This Been Tested?

• cargo test -p harper-core issue_2609 — 1 passed, 0 failed
• CARGO_INCREMENTAL=0 just test-rust — full Rust workspace test suite passed
• CARGO_INCREMENTAL=0 cargo build -p harper-core --all-targets -q — passed
• Release CLI against the exact reported sentence with only ItsContraction enabled — lint_count: 0
• just format and cargo fmt -- --check — passed
• Additional precommit components passed: dictionary audit, Clippy, all 43 cargo hack --each-feature combinations, WASM/JS/package builds, Harper.js tests (200), Obsidian tests (21), and VS Code integration tests (35)

The monolithic just precommit command was split locally because its generated Rust artifacts exhausted the available disk. The headed Chrome/Firefox suites also exposed unrelated local macOS launch-wrapper/UI timing failures; GitHub Actions remains the integration authority for those suites.

AI Disclosure

• I am a human and didn't use any AI.
• I used LLM features of my editor, but not an agent.
• I used an AI agent interactively.
• I am an agent or I got an agent to do the work autonomously.

If Your PR Implements or Enhances a Linter

• I made up the sentences in the unit tests.
• The sentences in the unit tests were generated by an AI.
• I'm using examples from the bug report / feature request.
• I collected real-world sentences for the unit tests.

Checklist

• I have performed a self-review of my own code
• I have added tests to cover my changes
• I have considered splitting this into smaller pull requests.