fix(passkeys): build signin metricsContext from flowQueryParams

[vpomerleau]

Because

• Passkey sign-in conversion cannot be measured reliably: the ceremony built its metrics context from a live location.search read, which the SPA can strip of flow params after page load. The resulting server login.complete (reason=passkey) then carried a flow_id that no longer matched the client passkey.auth_success, so the two events failed to join in analytics.

This pull request

• Sources the passkey metricsContext from the frozen flowQueryParams snapshot the app captures at load and feeds to Glean, in packages/fxa-settings/src/lib/passkeys/signin-flow.ts, so the server login.complete flow_id matches the client passkey metrics. This mirrors the existing SigninPasskeyFallback pattern.
• Threads flowQueryParams through usePasskeySignIn and its four callers (Index, Signin, SigninAlternativeAuthOptions, SigninPasswordlessCode), keeping the live location.search queryParams for post-signin navigation.
• Adds a regression test asserting metricsContext is built from flowQueryParams, not the live location.search.

Issue that this pull request solves

Closes: FXA-14093

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