ci: add java phase-2 conformance coverage

[avinxshKD]

Adds Java to phase-2 protocol conformance and wires existing Java regression tests into CI.

Changes:

• Added java to the phase-2 runtime set in test_protocol_conformance_phase2.py.

• Updated phase-2 matrix to include Java results for all existing cases in cross_runtime_matrix.phase2.json.

• Added a java-test CI job to compile and run TestLiteralEval and TestConcoredockerApi in ci.yml.

• Documented Java phase-2 tracking/execution in PROTOCOL_FIXTURES.md

Scope: Conformance + CI only (no runtime changes).

Varified:

1. pytest test_protocol_conformance_phase2.py -q -o addopts='' passed.
2. Java tests passed locally:
   TestLiteralEval
   TestConcoredockerApi

closes #520

[avinxshKD]

@pradeeban @Rahuljagwani @mayureshkothare pls take a look, kept this intentionally narrow to match issue scope: phase-2 conformance + CI wiring only, no runtime behavior changes.

[avinxshKD]

Also wanted your guidance on, should I prioritize:

adding a Java node end-to-end example, or
extending conformance matrix coverage beyond current parser/API cases?

[pradeeban]

Java end-to-end sounds good if you want us to stop calling it a toy. :D