Add API to set Wi-Fi frequency bands

[abeltrano]

Note: most of the work in this PR was done by @corbin-phipps in #121.

Type

• Bug fix
• Feature addition
• Feature update
• Documentation
• Build Infrastructure

Side Effects

• Breaking change
• Non-functional change

Goals

• Allow configuration of Wi-Fi Access Point frequency bands.

Technical Details

• Add protobuf+grpc API WifiAccessPointSetFrequencyBands.
• Update WifiAccessPointSetPhyType to return InternalError when unspecified access point operations fail.
• Make request validation helpers static.

Test Results

• All unit tests pass on Linux.

Reviewer Focus

• None

Future Work

• None

Checklist

• Build target all compiles cleanly.
• clang-format and clang-tidy deltas produced no new output.
• Newly added functions include doxygen-style comment block.