test: set Rust test thread stack size

[bolinfest]

Summary

Set RUST_MIN_STACK=8388608 for Rust test entry points so libtest-spawned test threads get an 8 MiB stack.

The Windows BuildBuddy failure on #18893 showed //codex-rs/tui:tui-unit-tests exiting with a stack overflow in a #[tokio::test] even though later test binaries in the shard printed successful summaries. Default #[tokio::test] uses a current-thread Tokio runtime, which means the async test body is driven on libtest's std-spawned test thread. Increasing the test thread stack addresses that failure mode directly.

To date, we have been fixing these stack-pressure problems with localized future-size reductions, such as #13429, and by adding Box::pin() in specific async wrapper chains. This gives us a baseline test-runner stack size instead of continuing to patch individual tests only after CI finds another large async future.

What changed

• Added common --test_env=RUST_MIN_STACK=8388608 in .bazelrc so Bazel test actions receive the env var through Bazel's cache-keyed test environment path.
• Set the same RUST_MIN_STACK value for Cargo/nextest CI entry points and just test.
• Annotated the existing Windows Bazel linker stack reserve as 8 MiB so it stays aligned with the libtest thread stack size.

Testing

• just --list
• parsed .github/workflows/rust-ci.yml and .github/workflows/rust-ci-full.yml with Ruby's YAML loader
• compared bazel aquery TestRunner action keys before/after explicit --test_env=RUST_MIN_STACK=... and after moving the Bazel env to .bazelrc
• bazel test //codex-rs/tui:tui-unit-tests --test_output=errors
  • failed locally on the existing sandbox-specific status snapshot permission mismatch, but loaded the Starlark changes and ran the TUI test shards