fix(network): fill iwd band and RSSI from Station diagnostics

[kvnloo]

Summary

On iwd connected refresh, call StationDiagnostic.GetDiagnostics(). When Frequency > 0, set frequencyMhz. When OrderedNetworks left signalStrength at 0 and RSSI is present, map dBm with existing signalToPercent. RSSI is read only via supported D-Bus integer forms (int16_t / int32_t), not int8_t.

Motivation

Origin issue #4148: the connection chip never showed band/RSSI on iwd because the backend never filled frequencyMhz / fallback strength even though the display path already existed.

Type of Change

• Bug fix
• New feature
• Breaking change
• Refactoring
• Build / packaging

Related Issue

Closes #4148

Testing

• Standalone iwd_diagnostics_test: exit 0 — 2437 MHz → 2.4 GHz, 5180 → 5 GHz, 5955 → 6 GHz; frequency 0 unlabeled; RSSI -70 → 60% only when OrderedNetworks left strength 0.
• Baseline mapping (-DIWD_DIAGNOSTICS_BASELINE) fails connected.frequencyMhz == 2437 (exit 1).
• Exact CI format gate over src/**/*.cpp and src/**/*.h: exit 0.
• Real iwd_service.cpp C++23 syntax-only with sdbus-c++ flags: exit 0 (confirms unsupported Variant::get<std::int8_t>() is gone).
• Full meson debug build not available on factory host; scoped tests above are the verifier evidence.

Checklist

• This PR is ready for review, or it is marked as Draft.
• I read and followed the relevant guidance in CONTRIBUTING.md.
• I ran just format with clang-format v22+ installed, or this PR has no code changes.
• I ran the relevant build or test commands, or explained why they were not run.
• I self-reviewed the changes.
• I checked for new warnings or errors.
• I updated user-facing documentation in docs/user/ when this PR changes documented behavior or configuration, or this PR does not require documentation changes.
• I added or updated assets/translations/en.json, or this PR adds no new user-facing strings.
• I did not edit non-English translation files unless this PR is explicitly for translation tooling, an import/export sync, or a maintainer-requested locale change.
• I used the existing canonical names for config keys, IPC names, paths, and identifiers.

Additional Notes

Prior PR #4240 was closed for an incomplete template body and had a debug-build failure on Variant::get<std::int8_t>(). This reopens the same branch at CI-fixed head 5de755e4135bce5c6c44bf47dc9f89a5ae49c1bb.