refactor: extract isWebviewIframe helper to reduce duplication in service worker

[harshagarwalnyu]

This is a follow-up refactor complementing #315495.

What

Extracts the repeated webview iframe pathname check into a shared isWebviewIframe(url) helper:

js function isWebviewIframe(url) { return (url.pathname === ${rootPath}/ || url.pathname === ${rootPath}/index.html || url.pathname === ${rootPath}/index-no-csp.html); }

Both getOuterIframeClient and getWebviewIdForWorkerClient (added in #315495) performed identical inline pathname checks. This extracts them into one place.

Why

The Copilot AI reviewer on #315495 flagged this duplication as a drift risk — if entry points change, both sites would need updating independently. This addresses that concern.

Relationship to #315495

This PR is stacked on top of #315495 and should only be merged after it. Happy to squash these into a single commit if the maintainers prefer, or @yongsooim is welcome to pull this change directly.

[harshagarwalnyu]

Closing — the suggestion is already captured in the comment on #315495. @yongsooim can incorporate the change directly if useful.

[Copilot]

Pull request overview

Refactors the webview service worker client-resolution logic by extracting the repeated “is this a webview iframe entrypoint?” pathname check into a shared helper, reducing duplication in the worker/iframe matching flow.

Changes:

• Extract repeated webview iframe pathname matching into isWebviewIframe(url).
• Reuse isWebviewIframe in getOuterIframeClient and getWebviewIdForWorkerClient to keep entrypoint checks consistent.
• Adjust resource/localhost request handling to resolve a missing webviewId for worker/sharedworker clients via getWebviewIdForWorkerClient.