[Repo Assist] feat: honour CancellationToken in provideDocumentFormattingEdits

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🤖 This is an automated draft PR from Repo Assist, an AI assistant.

Summary

The CancellationToken passed to provideDocumentFormattingEdits was previously ignored. This means that when VS Code cancels a format request (e.g. user makes a new edit, closes the document, or retriggers formatting), the extension kept waiting for phpcbf to finish — even though the result would be discarded.

Changes

Three improvements in provideDocumentFormattingEdits:

1. Fast path: if the token is already cancelled before format() is called, return [] immediately.
2. Cancellation listener: register token.onCancellationRequested to reject the Promise the moment VS Code cancels, so VS Code stops waiting even while phpcbf is still running in the background.
3. Race check: after format() resolves, re-check token.isCancellationRequested to catch cancellations that arrived during the phpcbf run.
4. Listener disposal: the cancellation listener is disposed in all code paths (resolve, reject, catch) to prevent event listener leaks.

// Before
provideDocumentFormattingEdits: (document, options, token) => {
    return new Promise((resolve, reject) => {
        phpcbf.format(document).then(...).catch(...);
    });
}

// After
provideDocumentFormattingEdits: (document, options, token) => {
    if (token.isCancellationRequested) {
        return Promise.resolve([]);
    }
    return new Promise((resolve, reject) => {
        const cancellationListener = token.onCancellationRequested(() => {
            cancellationListener.dispose();
            reject();
        });
        phpcbf.format(document)
            .then(text => {
                cancellationListener.dispose();
                if (token.isCancellationRequested) { reject(); return; }
                ...
            })
            .catch(err => { cancellationListener.dispose(); ... });
    });
}

Why This Matters

When a user types quickly while the formatter is running, VS Code cancels the in-flight format request and starts a new one. Without cancellation support, multiple format operations pile up waiting for phpcbf responses that VS Code will never use. With this change, VS Code's cancellation is respected and the Promise rejects immediately, keeping the extension responsive.

No Conflicts

This change is entirely within the activate() function's registerDocumentFormattingEditProvider block and does not touch any of the areas modified by the other open Repo Assist PRs (#48, #49, #51, #53, #56, #58, #60, #61, #63, #65, #68, #69, #73, etc.).

Test Status

• ✅ npm run test:unit — 7/7 unit tests pass
• VS Code integration tests are not runnable in CI without a display server (infrastructure limitation)

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