Friendlier loadTestConfig() error, point to example.json, gtest behind vcpkg feature

[TuxedoFish]

Summary

Closes out the two remaining items from #113 that weren't fully addressed in #115/#116: the loadTestConfig() error message itself, and the packaging note about gtest.

• Friendlier loadTestConfig() error message - v0.1.0 review: correctness issues and onboarding gaps #113 specifically called this out as "the single biggest barrier to picking the project up": the exception thrown when examples/test.json is missing was a bare Could not open config file: <path> with zero guidance. It now explains the file is gitignored/not shipped, points at the existing examples/example.json template, and links the docs.
• Point the docs at examples/example.json - this template file already existed in the repo but neither the README nor docs/examples.md referenced it; both now point at it as the thing to copy.
• gtest behind an opt-in vcpkg feature - previously an unconditional vcpkg.json dependency, so every downstream consumer built it even with HYPERLIQUID_BUILD_TESTS=OFF. Moved into a "tests" manifest feature, only requested when HYPERLIQUID_BUILD_TESTS=ON. This requires HYPERLIQUID_BUILD_TESTS's option() to be declared before project() in CMakeLists.txt, since VCPKG_MANIFEST_FEATURES has to be set before the toolchain's manifest-install step runs (which happens as part of project()) - the duplicate later declaration was removed.

Test plan

• -DHYPERLIQUID_BUILD_TESTS=OFF: confirmed via find build/vcpkg_installed -iname "*gtest*" that gtest is never installed.
• -DHYPERLIQUID_BUILD_TESTS=ON (both manually and via cmake --preset default): confirmed gtest is installed, and ctest reports 28/28 passing.
• Clean build with -DHYPERLIQUID_WARNINGS_AS_ERRORS=ON in both configurations - no warnings/errors.
• Actually reproduced the old failure mode (removed examples/test.json, ran rest_orders) and confirmed the new message prints as intended before the process aborts.
• mkdocs build --strict still passes clean after the docs/examples.md edit.

🤖 Generated with Claude Code

https://claude.ai/code/session_018Le73N2EZjGLsCqmCScAse

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.53%. Comparing base (678c335) to head (e7dba4b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #118   +/-   ##
=======================================
  Coverage   69.53%   69.53%           
=======================================
  Files          53       53           
  Lines        8219     8219           
  Branches      513      513           
=======================================
  Hits         5715     5715           
  Misses       2501     2501           
  Partials        3        3           

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