[mac] introduce ScanResult to encapsulate scan results (#12453)

This commit introduces the `ScanResult` class, which inherits from
`otActiveScanResult`. This new class provides C++ idiomatic getter
methods that return core OpenThread types (e.g., `ExtAddress`,
`ExtendedPanId`, `NetworkName`) instead of raw C structures.

The logic for parsing a received Beacon frame and populating the
result fields is moved into `ScanResult::PopulateFromBeacon()`.
This centralizes the parsing logic and allows it to be reused
across different modules.

Consequently, `Mac`, `DiscoverScanner`, `Seeker`, and
`PanIdQueryServer` are updated to utilize `ScanResult`. The `Mac`
class is also updated to use the `Callback` template for the active
scan handler, replacing the previous raw function pointer and context.

tests/unit/test_seeker.cpp

@@ -39,7 +39,6 @@ class UnitTester
 {
 public:
     using Seeker         = MeshCoP::Seeker;
-    using ScanResult     = MeshCoP::Seeker::ScanResult;
     using CandidateEntry = MeshCoP::Seeker::CandidateEntry;
 
     static void CreateScanResult(ScanResult &aResult, uint64_t aExtPanId, uint64_t aExtAddr, int8_t aRssi)