Add fuzzer for ZstdEncoder

[yawkat]

Parent: #231

Add a fuzz target for io.netty.handler.codec.compression.ZstdEncoder.

Scope:

• Create fuzzing-tests/src/main/java/io/netty/handler/codec/compression/ZstdEncoderFuzzer.java.
• Mirror the Netty target package: io.netty.handler.codec.compression.
• Upstream Netty source: netty/netty:codec-compression/src/main/java/io/netty/handler/codec/compression/ZstdEncoder.java.
• Suggested input surface: ByteBuf payloads, compression levels, block sizes, max encode sizes, and flush/close sequences.
• Follow existing handler fuzzer conventions: @FuzzTarget, fuzzerTestOneInput(...), and main(...) using LocalJazzerRunner.

Rationale:
Complements existing ZstdDecoder coverage and covers Zstandard encoder buffering and bounds handling.

Acceptance criteria:

• The fuzzer builds and runs locally.
• Expected Netty validation/decoder/encoder exceptions are handled consistently with existing fuzzers.
• The new fuzzer is included in the fuzzing test targets.