fix(pair): route in-app camera v2 scans to the v2 supplicant screens

[dschom]

Because

• Scanning a v2 QR with a mobile Firefox's own camera completed pairing, but walked the user through the legacy v1 screens.
• Both Android and iOS reach pairing through this entry point, so neither got the v2 flow.

This pull request

• Hands /pair/supp off to /pair/supplicant/connect_this_device only when the authority's fragment, the browser's fxa_status and pairing.version all report v2
• Waits on fxa_status before choosing a flow, and only for a v=2 fragment — v1 URLs are unaffected
• Sends the v2 post-OAuth reload to /pair/supplicant/sync_success rather than the v1 /oauth/success/:clientId
• Reuses the OAuth request already in the URL for v2, asking the browser via pair_oauth_start only when the URL carries none
• Adds unit coverage for the version gate and the param source

Issue that this pull request solves

Closes: FXA-14427

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: the version gate in pages/Pair/Supp/index.tsx, and getOAuthParams in pairing-supplicant-integration.ts
• Suggested review order: the Pair/Supp gate first, then the param-source change it depends on
• Risky or complex parts: an in-app camera scan reaches FxA through app-services, which rewrites the path to the v1 /pair/supp and copies the authority's fragment over verbatim — hence a v=2 marker on a v1 route. Reusing that URL's OAuth request is load-bearing: app-services holds the PKCE verifier for the code it expects back, so a second pair_oauth_start flow would strand it.

Screenshots (Optional)

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

Other information (Optional)

• No functional coverage added: every v2 pairing spec is gated behind an emulator or Nightly and skips in CI. AndroidSupplicant.beginPairing() drives this exact path if a real-device regression guard is wanted.
• Behaviour is unchanged wherever pairing.version is not 2, or where the supplicant does not report v2 pairing support.