Add GoogleTest regressions for vdf_client session parsing

[hoffmang9]

Summary

• move InitSession handshake parsing/validation into src/vdf_client_session.h so it can be reused in production and regression tests
• add vdf_client_session_test with GoogleTest coverage that now explicitly locks in both previously reported issues:
  • read-error handling is checked immediately after reading disc_size and form_size (so parse/validation does not run on truncated reads)
  • discriminant/form size bounds validation from 03e0b34967833fdb6e94921ad318ab081de2b747 remains covered (disc_int_size <= 0, oversized discriminant, zero form size, wrapped negative form size)
• integrate GoogleTest/CTest execution into the consolidated CI workflow and update docs (README.md, README_ASIC.md) plus ignore build-* scratch directories

Test plan

• make -f Makefile.vdf-client vdf_client
• cmake -S src -B build-regression -DBUILD_PYTHON=OFF -DBUILD_CHIAVDFC=OFF -DBUILD_VDF_CLIENT=OFF -DBUILD_VDF_BENCH=OFF -DBUILD_VDF_TESTS=ON -DBUILD_HW_TOOLS=OFF -DENABLE_GNU_ASM=ON
• cmake --build build-regression --target vdf_client_session_test
• ctest --test-dir build-regression --output-on-failure -R '^regression\.'

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Note

Medium Risk
Adds new build/test dependencies and CI execution paths (GoogleTest/CTest), which can introduce cross-platform build/runner issues even though production logic changes are small and localized to session parsing.

Overview
Adds a reusable InitSession implementation in vdf_client_session.h and updates vdf_client.cpp to use it, improving handshake parsing error handling (explicit failures on truncated reads) without changing the high-level session flow.

Introduces a new GoogleTest-based vdf_client_session_test regression suite (run via CTest with regression. prefix), wires GoogleTest/CTest into BUILD_VDF_TESTS CMake builds via FetchContent, and runs these regressions in CI on Unix and Windows. Docs are updated to describe the new test matrix and build targets, and .gitignore now ignores build-* scratch directories.

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[wjblanke]

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