Fix WireGuard rotation edge cases (#766)

[kasnder]

Fixes #766 — the four verified WireGuard rotation/profile-generation edge cases, bundled because each alone is tiny.

1. Provider resources orphaned when the relay fetch fails (the one with real user cost)

MullvadProfileGenerator.java:126-142 created the Mullvad device (fetchWebToken + createDevice), and IvpnProfileGenerator.java:133-146 minted the IVPN session, before chooseRelay(fetchRelays(), …) ran. A failing relay-list fetch therefore left a device/session attached to no profile.

This is the only item with an irreversible external cost: a Mullvad account has a hard cap of 5 device slots and IVPN caps concurrent sessions, so repeated failed setups burn provider quota, and nothing in the app can release the orphan — the user has to clean it up on the provider dashboard.

Fix: fetch relays and choose the relay first; create the device/session only once a relay is secured. Nothing after creation can fail (buildConfig is pure string building), so no cleanup path is introduced — deliberately, since best-effort cleanup here would risk masking the original error for no gain.

2. Write-only previous-key prefs retained old private keys

<provider>_previous_privkey / <provider>_previous_address were written in VpnKeyRotationManager.commitProviderKey (:349-353) and cleared on completion (:429-434), but read nowhere — rollbackProvider uses its in-memory parameters. They bought no recovery, while process death between the write and the clear left an old WireGuard private key in plaintext SharedPreferences indefinitely.

Fix: drop the writes and the now-dead clearPrevious/currentAddress helpers, and clean up any values already persisted by earlier versions in ApplicationEx.migratePreferences so existing installs don't keep a stale key forever.

Deliberately out of scope: implementing real prefs-backed rollback recovery (surviving process death mid-rotation). That is a much larger design change — it needs a durable rotation journal and a resume path on startup — and does not belong in a bundle of small edge-case fixes.

3. Blind /32 suffix (latent hardening)

VpnKeyRotationManager.java:433-438 appended /32 regardless of address family. The issue missed a second copy at IvpnProfileGenerator.java:317-321; both are fixed to emit /128 for IPv6. Latent today because IVPN currently returns IPv4, so this is correctness hardening rather than a live bug.

4. Malformed Address line for IPv4-less reusable profiles (latent hardening)

MullvadProfileGenerator.buildConfig (:354-357) emitted Address = , fc00:… when ipv4_address was empty — reachable via deviceFromConfig (:196-208) for an imported IPv6-only reusable config. TC's own parser tolerates it; standard wg-quick does not. Now only the non-empty components are joined.

Test evidence

New ProfileGeneratorTest (5 tests) and PreviousKeyPreferenceMigrationTest (1 test). The generators are exercised through small package-private seams (fetchRelays, fetchWebToken, createDevice, createSession, plus newPrivateKey/derivePublicKey so Robolectric never loads the native wgbridge library); production behaviour is unchanged.

• Item 1: mullvadRelayFailureDoesNotCreateDevice / ivpnRelayFailureDoesNotCreateSession assert the relay failure propagates unchanged and that fetchWebToken/createDevice/createSession were never invoked. Verified this fails on the pre-fix ordering: temporarily restoring the old order in MullvadProfileGenerator.generate gives 5 tests completed, 1 failed.
• Item 4: mullvadIpv6OnlyReusableConfigHasNoLeadingAddressComma and mullvadReusableConfigKeepsBothAddresses.
• Item 3: ivpnIpv6AddressGetsIpv6Cidr asserts Address = fc00:bbbb:bbbb:bb01::2/128.
• Item 2: obsoletePreviousKeyPreferencesAreRemoved seeds all four legacy keys plus unrelated prefs and asserts the migration removes exactly the four. The pre-existing VpnKeyRotationManagerTest assertion on mullvad_previous_privkey still passes.

./gradlew :app:compileGithubDebugJavaWithJavac -q                  # exit 0
./gradlew :app:testGithubDebugUnitTest --tests '*VpnKeyRotation*' \
    --tests '*WgProfile*' --tests '*Ivpn*' --tests '*Mullvad*' -q  # exit 0
./gradlew :app:testGithubDebugUnitTest -q                          # exit 0

Full suite: 262 tests, 0 skipped, 0 failures, 0 errors.