Make HPACK encoding test deterministic

[sainadh777]

Summary

• allow package-local injection of the HPACK header encoding policy
• make TestHpack.testEncode() explicitly force Huffman encoding
• remove the checked-in TODO and the test's dependency on the default length heuristic

Rationale

TestHpack.testEncode() asserts a 27-byte encoded result, which assumes Huffman encoding. The default encoder currently chooses Huffman encoding for values longer than five characters, but that heuristic may change. This implements the guidance in TODO.md by making the test select Huffman encoding directly.

The default constructor still uses DEFAULT_HEADER_FUNCTION, so production behavior is unchanged. The new constructor and the exposed nested interface remain package-private.

Verification

• ant test -Dtest.entry=org.apache.coyote.http2.TestHpack -Dtest.silent=true
• ant -Dexecute.validate=true validate
• complete unfiltered suite on Linux/JDK 21: ant test -Dtest.silent=true -Dtest.threads=4 -Dtest.openssl.path=/usr/bin/openssl
• clean source build: ant clean, then ant
• runtime smoke: started the generated distribution, received HTTP 200 from http://127.0.0.1:8080/, and stopped Tomcat cleanly