feat: add mainnet probe tests

[piotr-iohk]

Addresses https://github.com/synonymdev/bitkit-nightly/issues/9 from https://github.com/synonymdev/bitkit-nightly/issues/6.

This PR:

1. Adds a @probe_mainnet E2E spec for Android mainnet Lightning probe runs.
2. Restores a seeded probe wallet and waits for wallet, Electrum, node, and channel readiness before probing.
3. Resolves configured LNURL/Lightning Address targets into invoices from PROBE_TARGETS_JSON.
4. Invokes the Android devtools probe command through ADB, without using the Bitkit probing UI.
5. Writes structured probe artifacts and fails the run when any required target/amount fails.

---

Note

Medium Risk
Adds a new Android mainnet probe flow that fetches live LNURL invoices and executes ADB content call commands, which can be flaky due to network/app readiness and CI timing. Also changes build/output artifacts and timeouts that may affect existing CI runs and local workflows.

Overview
Introduces a new Android mainnet Lightning probe smoke test (@probe_mainnet) that restores a seeded wallet, waits for wallet/electrum/LN readiness, resolves targets from PROBE_TARGETS_JSON (Lightning Address or LNURL callback), fetches BOLT11 invoices, and probes them via adb shell content call against the app’s devtools provider.

Adds reusable helpers: waitForMainnetWalletReady (and refactors mainnet LN payment spec to use it) plus a probe module that validates target config, runs the probe command with configurable timeouts/retries/delays, and writes artifacts (probe-results.json + a markdown report, also appended to GITHUB_STEP_SUMMARY).

Updates tooling/stability: build-android-apk.sh now supports BACKEND=mainnet and writes a distinct mainnet APK filename, increases wallet restore “Get Started” timeout, sets Android Appium newCommandTimeout to 300s, and adds a small testing-guidance note in AGENTS.md.

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[piotr-iohk]

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