docs: clarify that GiacContext does not yet isolate evaluations

[s-celles]

Summary

The public signature giac_eval(expr, ctx::GiacContext) and the GiacContext type implied per-context isolation of variable bindings and computation state, but the C++ wrapper currently exposes only a process-wide singleton context. Every GiacContext value ends up funnelled into the same giac::context * (_get_cxxwrap_context()), so := bindings persist across every giac_eval call regardless of which GiacContext was passed.

This was surfaced by the MCP extension whose tool description promised "each call is independent" — a claim the implementation could not honour. Rather than masking the limitation with a restart; workaround in the MCP handler, this PR owns up to it in three layers of documentation:

• !!! warning admonitions in the docstrings of giac_eval and GiacContext pointing to the upstream tracker.
• A clarifying comment on _giac_eval_string explaining why its ctx_ptr parameter is ignored.
• An updated MCP tool description telling LLM clients that bindings persist and recommending purge(name) / restart to clear state.

Upstream tracking

Filed as s-celles/libgiac-julia-wrapper#3. Once the wrapper exposes a context-aware eval entry point, _giac_eval_string can pass ctx_ptr through, real per-GiacContext isolation becomes possible, the MCP server can do clean per-call isolation, and these warning blocks can be removed.

Test plan

• julia --project=. -e 'using Giac, ModelContextProtocol; include("test/test_mcp_ext.jl")' — 44/44 passing
• julia --project=docs docs/make.jl — no new warnings; the !!! warning admonitions render under Documenter
• CI green

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.86%. Comparing base (681f1ad) to head (3dd0122).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #32      +/-   ##
==========================================
+ Coverage   79.76%   79.86%   +0.10%     
==========================================
  Files          23       23              
  Lines        2891     2891              
==========================================
+ Hits         2306     2309       +3     
+ Misses        585      582       -3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.