fuzzer: add radius_encode harness to be consumed by OSS-Fuzz

[DavidKorczynski]

The goal is to increase code coverage of the folder here: https://storage.googleapis.com/oss-fuzz-coverage/freeradius/reports/20260515/linux/src/freeradius-server/src/protocols/radius/report.html

[claude]

Claude Code Review

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[alandekok]

There is already an unused encode stub in fuzzer.c. That code also uses the common.c file, which makes the protocol-specific fuzzers much simpler.

Since the encode path requires protocol-specific information, and the fuzzer.c file is now tiny, it might be worth just making protocol-specific encoders for many things:

• copy fuzzer.c to fuzzer_radius.c (which removes a lot of the boilerplate of fuzzer_radius_encode.c in this PR)
• poke the build system to treat fuzzer_radius as a separate target, but which is still FUZZER_PROTOCOL
• update fuzzer_radius to call the encode path as is done in this PR.

Once that's done, it should be fairly straightforward to do the same for the other protocols.