fix: crash when changing rgs server

[jvsena42]

Description

This PR fixes an ANR (Application Not Responding) crash on the RGS Server settings screen caused by catastrophic regex backtracking.

The URL_PATTERN regex had a nested quantifier (/[-a-z\d%_.~+]*)* that triggered exponential backtracking in Android's ICU regex engine. Since isValidURL ran on every keystroke on the main thread via computeState, certain URL inputs would freeze the app for 5+ seconds, triggering an ANR.

This PR:

1. Replaces the monolithic URL regex with java.net.URI parsing + simple hostname/path patterns (matching the approach already used in ElectrumConfigViewModel)
2. Adds URL reachability validation via UrlValidator before restarting the node, preventing invalid RGS URLs from being saved
3. Adds unit tests for both RgsServerViewModel and LightningRepo.restartWithRgsServer, including timeout-based tests to detect backtracking regressions
4. Add a 1 second debounce

Preview

Screen_recording_20260401_140552.webm

QA Notes

1. No ANR when typing URLs

1. Go to Settings > Advanced > RGS Server
2. Type https://rapidsync.lightningdevkit.org/snapshot character by character
3. Expected: No freeze or ANR, UI remains responsive

2. Invalid RGS host shows error toast

1. Enter https://rapidsync.lightningdevkit.org/snapdsadaDSF as RGS URL
2. Tap Connect
3. Expected: Error toast appears (URL unreachable), app does not crash

3. Valid RGS host connects successfully

1. Enter the default RGS URL or another valid one
2. Tap Connect
3. Expected: Success toast appears

4. Unit tests

• ./gradlew testDevDebugUnitTest --tests "to.bitkit.ui.settings.advanced.RgsServerViewModelTest" — 19 tests pass
• ./gradlew testDevDebugUnitTest --tests "to.bitkit.repositories.LightningRepoTest" — all tests pass including 4 new RGS tests

[jvsena42]

jvsena@Joaos-MacBook-Air ~ % ping https://rapidsync.lightningdevkit/snapshot
ping: cannot resolve https://rapidsync.lightningdevkit/snapshot: Unknown host
jvsena@Joaos-MacBook-Air ~ % ping https://rapidsync.lightningdevkit/snapshot/0
ping: cannot resolve https://rapidsync.lightningdevkit/snapshot/0: Unknown host
jvsena@Joaos-MacBook-Air ~ % 

LDK RGS is not reachable for me

[jvsena42]

the correct address is https://rapidsync.lightningdevkit.org/snapshot/