test(docker): enable Python→C# integration test in cli image

[G00dS0ul]

Description

Re-enables the C# section of the Python integration test in
tools/docker/tests/scripts/index.py, which was previously commented out.

The test loads Sum.cs via the C# loader and asserts:

• sum_cs(3, 4) == 7
• greet_cs('MetaCall') == 'Hello MetaCall from C#!'

This PR is expected to FAIL in CI. It is intentionally opened to surface the
cs_loader init failure in the metacall/core:cli runtime image so we can track
and fix it separately (coordinated with @viferga).

Root cause of the failure

On C# loader init, netcore_linux::ConfigAssemblyName() builds the Trusted
Platform Assemblies list by iterating the runtime/assembly directory, which is
configured to /usr/local/metacall/build. That path exists in the dev/build
image but not in the stripped runtime/cli image, so the directory scan
fails.

Decoded stack trace from the published metacall/core:cli image:

terminate called after throwing 'std::experimental::filesystem::filesystem_error'
  what(): directory iterator cannot open directory: No such file or directory [/usr/local/metacall/build]
#12 cs_loader_impl.c:236   cs_loader_impl_initialize
#10 netcore_linux.cpp:247  start
#9  netcore_linux.cpp:87    ConfigAssemblyName
#8  netcore_linux.h:100     directory_iterator  → "/usr/local/metacall/build"
#6  __cxa_throw → #5 std::terminate() → #2 abort

Notes:

• The published :cli image predates Fix cs_loader process crash on missing TPA directory (#789) #831, so it still hits the throwing
  directory_iterator (hard abort / SIGABRT). With Fix cs_loader process crash on missing TPA directory (#789) #831's
  AddFilesFromDirectoryToTpaList error_code guard now on develop, a cli image
  rebuilt from source instead fails gracefully: metacall_load_from_file('cs', ...)
  returns false and the Python assertion fails cleanly. Either way the test is red.
• The real fix (separate PR) is to point the C# assembly search path at the
  actual install location in the runtime image instead of the build dir.
• index.js contains the same commented-out C# block; left commented in this
  PR and deferred.

Relates to #123 (Make C# Loader production ready).

Type of change

• New test (adds/enables test coverage)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change

Checklist

• I have performed a self-review of my own code
• I have commented my code where necessary
• My changes generate no new warnings
• This change requires follow-up work (cs_loader install-path fix) — tracked under Make C# Loader production ready #123