bug(settings): Stop duplicate sign-in verification emails for dormant accounts

[dschom]

Because:

• Dormant accounts signing in to Settings/Sync received two identical OTP verification emails. The auth-server sends one during signInWithAuthPW, and the shared handleNavigation sent a second via sessionResendVerifyCode (added in 2fce100 to fix a cached-flow duplicate, but it also fired in the password flow where the server already sent).

This PR:

• Gates the client-side sessionResendVerifyCode in handleNavigation behind a new optional sendVerificationEmailFromClient NavigationOptions flag.
• Sets the flag only in the cached-signin paths (SigninCached and the prompt=none Authorization container), the only flows that perform no server-side login and therefore must send the OTP client-side.
• Leaves the flag falsey for the password/unblock/reauth paths, which already send server-side, eliminating the duplicate email.
• Adds regression tests covering both the gated (cached) and non-gated (server-login) paths so the duplicate cannot silently return.

Issue that this pull request solves

Closes #FXA-14109

Checklist

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• 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)

To test manually we must have an 'old' account that goes into the 'must_verify' state.

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