test(passkey): add PRF output support to the virtual authenticator

[vpomerleau]

Because

• Nothing in the test stack could produce a PRF output, so the kB wrap work in this epic could not be tested above the unit layer.
• Two consumers were blocked on it: auth-server integration tests, and functional tests whose polyfill delegates its crypto to this authenticator.

This pull request

• Returns a deterministic per-credential PRF output at assertion, keyed on the eval salt the server sent.
• Reports prf.enabled at registration, the capability flag the server records.
• Models an authenticator with no PRF support via prfSupported: false, for the password-fallback path.
• Surfaces the output to page code through the Playwright polyfill as an ArrayBuffer.
• Drives the adapter spec's two prfEnabled cases through the real path, retiring its hand-patched clientExtensionResults.
• Moves the virtual authenticator behind a new @fxa/accounts/passkey/testing entry point.

Issue that this pull request solves

Closes: FXA-14268

Checklist

Put an x in the boxes that apply

• My commit is GPG signed.
• If applicable, I have modified or added tests which pass locally.
• I have added necessary documentation (if appropriate).
• I have verified that my changes render correctly in RTL (if appropriate).
• I have manually reviewed all AI generated code.

How to review (Optional)

• Key files/areas to focus on: libs/accounts/passkey/src/lib/virtual-authenticator.ts
• Suggested review order: a4fdafaa35 (entry point) → authenticator → polyfill
• Risky or complex parts: all test-only; derivePrfOutput is deterministic by construction and is not a real KDF

Screenshots (Optional)

Please attach the screenshots of the changes made in case of change in user interface.

Other information (Optional)

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[Copilot]

Pull request overview

Copilot reviewed 11 out of 11 changed files in this pull request and generated 1 comment.

Suppressed comments (2)

libs/accounts/passkey/src/lib/virtual-authenticator.ts:270

• A PRF-capable registration still emits flags UP | AT | UV only and appends no authenticator-extension CBOR. That contradicts the stated hmac-secret registration support and lets tests produce prf.enabled: true without the corresponding ED flag and hmac-secret: true authenticator output. Set ED and append that CBOR output when PRF is requested and supported.

    const flags = Buffer.from([0x41 | uvFlag]); // UP + AT (+ UV unless suppressed)

libs/accounts/passkey/src/lib/virtual-authenticator.ts:149

• The advertised per-UV-state derivation is explicitly not implemented here: userVerified never reaches prfSecret or derivePrfOutput, so UV and non-UV assertions return identical PRF bytes. Either incorporate UV state into the derivation and cover both states, or update the PR contract to document that this test authenticator intentionally models only the UV output.

 * Not keyed on UV state, unlike CTAP 2.1: FxA requires UV at both ceremonies
 * and rejects a no-UV assertion server-side, so there is no reachable no-UV
 * output to model.